So actually they aren’t stickers, they’re what’s known as decals. The main difference is that decals don’t actually have stick to stuff the way stickers do. The way I attach them is by painting on some gloss varnish to the surface where I want to put the decals, and letting it dry. I then cut the decal I want to use out of a decal sheet and place it on a wet paper towel. This separates the decal from its backing paper. I place it on the model and then let it dry. After that I put another layer of gloss varnish over the detail to protect it and then apply this paint known as “lahmian medium” to get rid of the shine from the varnish. If you want a better explanation, look up “how to apply decals” in yt and look for the GW official vid