I want to replace the Obi-Wan head with another human head. Can I transfer the head from a picture into the skin? How do I make the head of the skin? Also just in general, how do you skin humans?

Like you'd reskin anything else. Just paste a different face over the original face. Make sure the eyes are at the same spot as the original texture's eyes, same goes for mouth and nose. Note that many human skins mirror the face, so watch out with weird shadows and stuff.

so just you'd just pop the picture over it?

yes.

What I find works best (using Paint Shop Pro X) is that I put the skin as a second layer, and put the skin with the face i want in the background layer under the original skin. I align the faces so they are in the right spots, and then I erase the face from the original skin on the one layer, so the other layer shows up. If there are any blank spots, I use the smudge tool to spread the face out and make it look better. A good example of this is here: