I start with this basic skull, which came with a handy stand which makes things a lot easier. This one used to have some stuff inside that made it "talk", but I took all that stuff out.
Then I cover it with strips of newspaper, about three layers. I have found if you alternate the colors, it helps you keep track of what you have done. For instance, use black and white for one layer, then something like the comics for the next layer, then black and white again.
After this dries, usually overnight, I cut from below where one ear should be up and across the top of the skull and down the other side. Then start working on peeling off the pieces. Once you get them off, tape up the inside really good and fill it with expandable foam (I used Great Stuff). Be careful not to put to much in at a time as it will make the head kind of shaped wrong, which could be a good thing if you're going for that look. I usually put in a few layers at a time to prevent this, or have poked holes into the foam when it is about half setup to let the gases escape from inside, which will stop it from expanding.
I think this ended up being the head for this guy...
One of the cool things is that every one of these will end up looking different. I also do not add teeth, I think they look kind of cool without them and I was probably too lazy to make them. I also followed Spooky's groundbreaker corpse tutorial to make these guys.
Spooky Blue's Tutorial