I'll post here what I tell folks that buy my decals:
Cut out the decal, paper and all, till you have an irregularly shaped piece of the paper with the decal on it. I don't trim too, too close because I don't want to mess with corralling little bits. But cut away the paper you aren't going to use.
Follow any instructions you have for the soaking very closely. The ones I make take only around 10 or fifteen seconds in the water, and it seems like the decal holds together better that way. Then place the decal - backing paper and all - in position and slide the paper out from under it, gently holding the decal itself close to where you want it to be.
You kinda have to manage the whole thing with one hand while pulling out the paper with the other - I found it works well to slide about an eighth to a quarter of an inch off the backing and onto the position first, then withdraw the rest of the paper, while gently holding the decal film itself in place
You can't move it around too much after that. Some, but not a lot of play, enough to get everything straight and in position. I like to have a wet, medium (1/2 inch flat watercolor brush) brush to use to gently smooth out any bubbles. And that's it.