This illustration enters the series of sketch exercises. Here are the steps for creating this sketch from a 3D skull rendering :
This is the rendering image that served as the basis for the sketch.
The first step lays the big bases of the sketch, with a pretty big brush.
The second step specifies the shapes and different parts of the object.
The third step is the longest, the realization of all the little details. In addition I have delimited the gray areas because the sketch is used for an exercise of values thereafter.