My latest painting is on the skull of a buffalo that belongs to a friend.  I have only had the skull for 2 years, thought it would be a good thing to finish and get back to Joel.
After talking to artists who have painted on skulls, I prepared the skull by cleaning it and then spraying it with a clear acrylic spray paint.  After it dried, I used acrylic gesso on the area (forehead) that I was going to paint the major subject - white buffalo portrait.
I used for reference one of my photos of a large Yellowstone bull.  I had to wing it for the coloring of the white bull as I have never seen a white buffalo and so I kept the head darker for contrast against the blue sky coloring and white values for the body.  I then painted the nostril bones red to tie it all together.