I just found a great tutorial for molding a plastic skull and casting plaster copies. There's lots of pictures and detailed instructions. The most expensive part is the liquid latex (if, of course, you already have a nice skull at home), but the 20-30$ it'll cost you in materials is well worth the massive amount of skulls you'll be able to make!
from The Creepy House Next Door, which also contains many more interesting tutorial revolving around creepy halloween yard decoration.
For painting the skull, I strongly suggest this step-by-step demonstration, by Dave the Dead from The Shadow Farm. He uses an urethane skull, but there's no reason why it wouldn't work on plaster. I'd suggest applying a coat of sealer to the plaster first, however, so it doesn't soak up all the paint.
A Link Dump Runs Through It
3 hours ago