Mask painting help

Hello everyone, I need to apply mask to hide specific part of the body on my character mesh, right now i do it with Photoshop with a black and white texture, but its difficult to be precise , and something i need a lot of try and error to get the correct area. Is there a way to do it with more precision?
I not a artist but something like that with help:

I think its call UV mapping? I just dont know how i can do it.
Thank !

maybe it is not best method, but it works: