(Quick 5min sloppy edit and also spooky color job)
Skulls, in actuality, have a lot of planes and are pretty complex and for this reason you might want to use more reference. But to successfully capture the bony essence of a skull you're gonna have to use planes, just simplified down and with toyed/cartoony proportions.
Also try to envision lighting as based on these planes, which face towards the light, face it less to varying degrees, or completely don't. Thinking of a box or a simple primitive under basically the same lighting situation as your image is a good place to start. Which sides catch the light, and how much of it? Now apply that to whatever you are drawing. A good but basic example of this is the nose ridge in the edit and in many side-lit pictures of skulls. (When drawing rounder objects, planes can be smoothed out, but are still the most reliable, solid basis to start with.)
