As a trans person, I'm not the biggest fan of these stickers. I just don't think it will change any minds of transphobes, seems more likely to make them angrier and more prone to ask if a person should be in... "their" bathroom.
It's not my intent to tone police or criticize, everyone should do as they see best. I've seen these irl and it's not like I tear them off, I just think they are more self gratification than activism. But even saying that, in these fucked up times, any trans joy is worthwhile.
I like to be sneaky. I dont want staff removing them. So I usually do the underside of the toilet seat (for mens room or unisex bathrooms), or the side of the toilet roll holder that faces the toilet.
Get a superglue bottle with a brush and coat the surface in that before putting the sticker on it. It'll make it a pain to remove and basically impossible without surface damage.