I just thought I'd share this. I learned it from the idrawgirls.com tutorial I've downloaded.
It's basically about face "planes", and about what value goes where. I hope you understand my crappy handwriting.
This is how I understood it, it might not be right or anything, but it was still helpful for me. I painted over the portrait on the right
and then used it to paint the portrait on the left, if you understand.