I used Spooky Blue's technique as a base plan for my new witch heads. I started with a Bucky Budget Skull and took all the hardware off. I then wiped the entire skull with WD-40 as a releasing agent. If you don't want to cut the dried mache off leave an open seam that starts at the base of the skull where the ears would be that reaches up the crown. Then I paper mached the back half or occipital part of the skull separately from the frontal part with the maxillary-facial portion. The whole process makes it much easier to lift the mache from your 20$ very nice skull. I used no glue in the first layer to aid in easy release. It is really hard to get it to stick to the skull, but it came off very easily once dried. You have to be very careful using that technique. I also mached the mandible on it's own to make things easier. I then taped the pieces of upper skull together and started filling the voids with expanding polyurethane foam.
Take your time with the spray foam (which is found near the plumbing supplies at the local hardware store). If you apply too much, the whole thing will blow apart.
In order to be able to put in eyes that glow, I placed two lengths of aquarium tubing through the eyes to the back of the skull for later wiring. The tubes will allow me to wire two LED's in each eye and have the switch at the base of the head, or wherever is convenient. If I don't get around to wiring the eyes this year, I won't have to make new heads next year.
I then sprayed more foam in to fill the skull. Note that I keep the tubing near the sides of the skull to allow for room to drill a socket for fixing the skull to the torso.