What is the best way to learn a new language as an adult?

I just had to learn French in middle age, and it's been fun. They key takeaways from my experience:
Contact is everything. The longer you spend listening, reading, speaking, just in general interacting with the target language, the better. Doesn't matter what you do - Duolingo or PeerTube videos, novels or comic strips.
Communication is the goal, not fluency. You can get the gender of a word wrong and people will still understand you. You can use the wrong tense and that's usually okay. Don't try to "sound more like a native" or "learn slang words that everyone uses," because heaven knows nobody is going to take you for a native. But if you can get the point across and can understand what people are saying, you win.
Speaking is 10x harder than listening or reading or even writing, because it involves not only forming sentences in an unfamiliar language, but also saying them, which involves your muscles. At first, it's really hard to say the sounds of the language that don't exist in your own language, and I found that very frustrating.
Language and culture are different, but interconnected. You don't really speak a language if you don't understand the culture it's attached to. For instance, at first I didn't know what the cashiers were asking me at the checkout, until I learned that they want to see the bags you brought from home to make sure they are empty. The problem with missing cultural references is that everybody around you knows them, and they don't understand why you don't, or what there is to explain.
One of the very few great use cases of LLMs is, in fact, talking with a chat bot. You give it a good prompt (look for them online) and you are forced to talk in the target language. If the bot can understand you, a native speaker probably will, too. A good tip is to try an AI conversation on the topic of something you are about to do in real life, like applying for an apartment or having a conversation about cheese.
Personally, I found that my language skills drowned completely under certain, specific circumstances. For instance, for the life of me I cannot understand voice messages, at all. Even phone conversations are really bad for me, both in talking and listening. I can have a perfectly fine conversation with someone, but when I have to talk with them on the phone, it's like I never learned the language.
The tool you use is not as important as the time you spend. Duolingo was really meh: too much useless vocabulary, not enough grammar and pattern recognition, lack of ability to specify areas of interest, down to always on animations even when you had them all turned off. But, despite the heavy focus on the words, "chouette" and "trousse," I sort of learned French to the point where I can follow everyone along and can speak and be understood. Took a year to the day and the entire tree.
- En francais, s'il vous plait.
  
  oN eSt En FrAnCe, On PaRlE fRaNçAiS !

Study the basics, consume media in said language. Use the language not caring if you do it wrongly. If people correct you, use their input to improve. The trick is getting over the "omg this is embarassing" hinge, when you are butchering it while learning.
- But keep in mind that native speakers might have an even worse understanding of grammar than foreigners: most locals speak intuitively and never care to actually study their native language.
  
  Yeah, but I count being able to communicate with natives a W. Even if it's not gramatically 100% correct.
- We live in not-our-native-language country, and I try to fail at using the local language multiple times per day. If it's an easy convo and I succeed it doesn't count for me.
  It's nice, pushes my language use out of its comfort zone. And when I fail I apologize and try to regroup. Definitely has resulted in some miscommunication lol but it's gotten me far fast.
  I've seen so many people here who know a LOT and are so scared to use it since it won't be "right"

The absolute best way is immersion. Full on survival, sink or swim, daily brain exhaustion to cram information in that you will use, over and over.
Short of that, finding ways to practice using the language is the key. Listening to videos is fine, but you need to simulate thinking and responding to make the language part of what your brain goes to. Find people online to talk to via Zoom or discord. I like to think of conservations I have and translate them in Google and re-run the interaction 4 or 5 times in the second language.
For numbers, find videos online that are things like lottery draws.
Bon chance!

Start dating a native speaker of the language :D
But seriously, find a method that works for you. Start by learning common vocabulary, learning grammar rules, and finding a place to practice a language. That could be an instructor, class, online community, or pen pal. There are sites for the latter. Read books and newspapers in your target language and add their subtitles to the films you watch.
- There was an old saying that the best way to learn a new language was 'under the mosquito net'

I read many great approaches here. One thing that I have the impression it was not mentioned is song lyrics. It's like syntax and vocabulary get somehow absorbed or something. And it's fun!
- Song lyrics are difficult (for me). I have difficulty understanding song lyrics even in my native language sometimes.
  
  Fair enough! Now that I think about it, maybe I should have specified that this is an online approach. It includes finding them, reading them and having a dictionary for translating them. Then, at some point, you learn them by heart. Also, I don't think that just doing this is enough, but it somehow speeds up the initial process, especially for speaking.
  
  Any nowadays there's so much slang and fake words thrown in that it's probably more harmful then good when learning languages. At least with current music anyways.
- I learned most of the German I know through German music and a little bit of Spanish from Latino music
- Yes! Not so much for beginners, but you can practice translation by translating songs on lyricstranslate.com. It's good fun!

See if there are evening classes you can sign up for in your area. This has a couple of benefits over pure self-study: firstly, if you don't really know any other languages yet, having a teacher to explain new concepts for you will be a big help. Secondly, it keeps you accountable if you need external motivation to keep at something. It can also be a good way to meet new people!
If you're going with self-study, look up what textbooks people recommend using for self-study for your target language (i.e. not one that relies heavily on having a teacher explain things for you). Obviously a textbook alone is not enough to learn a language, but imo it's the best way to get to grips with the basics of grammar before you move on to more advanced stages. There are also plenty of online communities to help you out with any questions, especially for more popular languages.
Finally, do NOT expect to make worthwhile progress with Duolingo or apps like that. They are not designed to help you master a language. You might learn some useful vocab from them, but if you're serious about learning a language, don't waste your valuable time.
- I 100% agree with your advice.
  I tried learning Portuguese with Duolingo, in a couple of months I only managed to learn words, but no grammar. I fared better with Babel, which has more structured courses, but since there was no accountability, I wasn't diligent and did not progress much. Finally, I enrolled in an evening class and in the span of 4 months my level went from "I barely know the basics" to A1.2
  Also, as others mentioned, consuming content in said language and conversing (even online) helps a lot (that's how I honed my English), having a pen pal is even better.

Language Transfer is free with no ads or intrusive permissions needed. The material is presented in a way that makes learning the language more natural feeling.
https://www.languagetransfer.org/
- I second this. It also teaches without using rote memorization. Better than any other books or videos I have ever seen. I have been supporting LT with a monthly donation for several years now.

If you are coming from another language, say C or Python, try https://tourofrust.com/
"The book" is always a good option. I like the variaton of Brown University: https://rust-book.cs.brown.edu/title-page.html
(I know that's not what they meant, I am just preaching the ghospel, here)

I use flashcard apps such as Anki.

I've been learning languages for the past 10 years or so, including English, the language I'm writing this message in, which is not my native language.
I certainly don't know what the "best" way is, but what I do is get a lot of input, review vocabulary with Anki (spaced repetition software), and slowly make my way through the pile of grammar.

As far as I can tell, nothing beats persistence. Ideally persistence out of necessity, like immersion, although that might be hard to achieve.
It's not really a thing you can shortcut.

For those looking to learn Spanish, find her:
https://youtu.be/G3R4-lj4aaQ

Seconding language transfer that someone mentioned, I found it to be hands down the best free resource for Greek.
Assimil courses are also some of the best materials out there, I highly highly recommend it if you can find a course for your target language.
As for apps, I use Clozemaster for drilling words (I use a premium account), but you can make your own anki decks for that purpose.
But nothing beats talking to native speakers, either using something like italki for actual lessons, or just finding random native speakers. I often forget words I repeated 1000 times during my studies, but words I learned by speaking to others somehow just get glued to my mind.

Learn the basics. This is the easy part and you should be able to make good progress. Find a textbook or good online resource that covers things like the alphabet, pronunciation, basic Grammer, etc.
Practice and accumulate vocabulary. You can do this basically however you want, but I would recommend the tool Anki (https://apps.ankiweb.net/), but you can use Duolingo or whatever. Learning vocabulary is key to being able to understand anything. Practice and repetition is key to gaining speed and fluency.
Use the language. Talk to people. Talk to yourself. Watch videos. Read. Play games. You figure it out, but actually using the language is how you grow and get better at it.

I'd say it depends on the time you're willing to spend. If you have all the time in the world, I would use a full immersion method, that's what works best for me. Put in as many hours of movies, podcasts, tv shows and Youtube videos as you can, in your target language, even if you don't understand a single word at first. Combine that with daily grammar lessons so you can get a grasp of that language structues. Comprehension will slowly grow day by day. With this type of method I was able to learn italian in 2 years, and japanese in 3 years, both at a very high level.
If you don't have much time to learn, you can always use a more traditional method. Find a course where you can practice 2-3 times a week with a teacher, and combine with 20-30 daily minutes in an app like Duolingo so you can also stablish some vocabulary. With this route you should achieve a decent level in your target language in about 4-5 years, at least based in my personal experience, as this is how I learnt english.
Remember learning a language is a marathon, not a sprint, and the only way of making progress is practicing almost every day. The single thing I would advice against is to study using ONLY apps like Duolingo or similar. They will award you with a false sense of progression, while in reality you will only be learning premade sentences in a vacum, that won't make you improve in real conversation enviroments. An app will always be lackluster in comparison with a real teacher or course.

need an excuse , time and place to learn one.

Hire a tutor or sign up for a college course. Professional teachers will always be better than most anything you can do on your own.

This can depend on your personality type and self-discipline.
If you're a methodical introvert who likes reading, go for a vocab-focused app with spaced repetition, and continue by reading actual newspapers or books (even children's books) as soon as you can.
If you're an impulsive extrovert with a short concentration span, you're going to need a more human experience, if possible 1-to-1 tutoring.
In all cases, don't get hung up on grammar and (especially not) phonetics at the start. Vocab is the key that will unlock the rest.
FWIW I speak multiple languages fairly competently and I'm a qualified English teacher.

Im still not great at Spanish because I dont practice frequently but these things helped me:
Learned alphabet.
Learned examples of conjugation of verbs with real life examples.
Started with children level books and increase difficulty to read and comprehend what is being said.
Listen to music in that language and learn what its saying.
Watching telenovelas with English captions and try familiarizing words
Final phase I think is visiting a country who's primary language is that language.

Practice. And then practice more And more And even more. Unlike all the shame people give it, I still think that Duolingo is a good tool. But, Duolingo on its own is not enough. Without using the language you won't get it.
I used Duolingo for Italian French czehc and Greek. It is enough for me to allow me know what an article is about in these language. If you want to get to conversation level, be ready to embrasse yourself. Without willing to make these mistakes, no matter what you do, you won't manage the language.
For context, my experience: Italian: started with Duolingo, eventually hired a teacher to improve my speaking level. I'm not perfect, but I manage to joke with the casual Italians I meet. French: used almost only Duolingo. There are however plenty of books and films and shows in french. I was in Paris a year ago, communicated almost solely in french and managed to do everything I need (including taking a friend to a clinic after he hit his head) Greek: started with Duolingo and hired a teacher for a while. It is hard to find communication partners and material so my level is not as high as I would like. To manage reading an article I need to concentrate very hard. Czech: still basic. I'm only half way through the Duolingo tree. I'm searching for other rmaterials, but I'm mostly lacking the time to dedicate myself to the task.

After learning the basics, try reading manga/comics that you are already familiar with, but in your target language.
The pictures help and you can read it at your own pace unlike listening to audio or watching videos.
I've read the entire Dragon Balls manga in Spanish.

There are lots of great things to do, but most important is to build a habit and keep working at it. Engage with the language every day, and work up to consuming content designed for native speakers.
Duolingo and similar apps are helpful, but don't put all your eggs in that, or any single, basket. Also do Anki or some other flashcard thing. And at the very least a beginner textbook or grammar guide is good to have. Early on, mix time between textbook, apps, 'comprehensible input' videos. Work your way up to reading, and look for graded readers to get started.
(Also, as Duolingo gets worse, consider alternatives like Memrise and Mondly. Or even paid stuff like Busuu, Lingq, Pimsleur, Babbel, Rosetta Stone. Also, your local library may give access to Transparent Language or Mango. Although of course I can't vouch for most of these personally.)
Another rec: check out !languagelearning@sopuli.xyz for a cool community. Good place to ask questions and get support.
There are probably a ton of cool learning resources specific to your target language. Look online for communities around it.

After getting a hang of the basics watch, listen to and read stuff (ideally also talk) in the target language. It's really that simple; yuu use the language, you'll learn the language.

Duolingo + language learning Books