Step 1. I printed out a skull papercraft from ravensblight.com at 150% size, cut it out of cardboard and hot glued it together.
Step 2. I started building up the eyebrows and cheekbones using crumpled paper, and covered the entire thing in paper mache.
Step 3. I cut out the areas for teeth, eyes, and inside the nostrils. Covered the whole thing in spackle and started sanding it down. It’s hard to tell on the lower jaw, but much of this was sculpted in roughly the same way as human skulls are varied in thickness.
Step 4. Sanded out to decently smooth texture, sculpted the teeth on (this was the hardest part). Painted it in a flat kelly green.
Step 5. Contoured the skull with a mixture of dark green and black pastel, and painted the red cheeks and gold tooth in yellow and gold dust. Covered teeth in gloss. Taped in eyes and nostril backing from the inside of the skull, stapled elastic on inside to attach the lower jaw to upper skull.
When Caliborn’s upper torso was revealed just before AX 2012, I repainted the teeth and put in new eye backings to reuse the helmet for Caliborn cosplay. The spirals were painted onto painter’s tape and taped over the existing cheek circles. At this point his giant eyelashes and sparkly eyes hadn’t been revealed. (I added them later but the eyelashes never stay on well.)