It's not too hard to do, just takes a while when you first try it. Just make the pixels in the middle the base color, then get a slightly darker color, and surround the base in that color, but leave out the corners. (sorry if I'm sounding a bit complicated) Just keep repeating this, and it should end up sort of like the skin I uploaded (it may not look the best the first time you try it, but believe me, this was like my fifteenth try at shading a skin) and if this doesn't make sense, I'll see if I can explain it a bit better, maybe I'll post something about how to do it (if I can figure out how, :P I'm sort of a noob to PMC)