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.

Rendu 3D crâne pour entrainement
The first step lays the big bases of the sketch, with a pretty big brush.

Digital Painting Croquis Crâne étape 01 Lady Melusine
The second step specifies the shapes and different parts of the object.

Digital Painting Croquis Crâne étape 02 Lady Melusine
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.