1. After careful analysis, draw as many skulls without reference as you can on one page. (Around 8-10)
2. Then check the reference to compare what you were missing. (Is the cranium in the right place compared to the jaw position? Is the nasal cavity too high? Too low? Is the mouth coming out too far, or not far enough?)
3. Then do one more page again without reference.
4. Then check the reference again to look for inconsistencies (it's OKAY to miss a lot of things!). 
5. Repeat this until you have 10 pages of skulls. Your final page will very much look better than your first.

HARD MODE: 

Draw 10 skulls, but STYLIZE the proportions. See if you can stretch and squish the proportions; elongated skull, big cranium, small mandible, so many combinations are possible! 

EPIC MODE: 

Paint those 10 skulls with a different angled light source for each. 

GOOD LUCK