Is there a skinning tutorial on GIMP or Paint.NET?
As far as I'm aware there isn't.
I was taught to skin on paint shop pro by FLASH_22798, from what I can tell, paint.net seems to be a little similar to that. I'm trying to learn on paint.net right now so that I can at least do something. But if you find a tutorial for paint shop pro, it might help you out a little.