The first step in building our skulls was to paper mache a balloon. Make sure to check the size and shape of your balloons ahead of time. I had one class where each student received a balloon that could be blown up to the size and shape of a basketball! The ideal shape is oval, and the size is up to you.
After the paper mache balloon dried completely, we poked holes into the areas where we wanted to insert egg carton cups (I'll use the word "cup" for lack of egg carton terminology) for eyes and a nose. If you look carefully, you can see the egg carton insertions in the first photo. You can also see where we taped egg carton cups for the cheek bones. Below the cheek bones you can see where we taped thin, pre-cut cardboard pieces for the jaw.
After the paper mache completely dried, we applied a coat or two of white gesso to strengthen our skulls. White acrylic paint works, too.
If you look carefully at the pink and purple skulls in the photo below you can see a shiny glitter coat that some students applied to their completed skulls. Excellent work, everyone!