from line drawing to shaded drawing

Nothing beats the observed detail of a live model with a decent, interesting lighting source. Even a photo taken with a nice light will be easier than 'fudging' a light source and the associated lights and shadows on the face. 

Take a simple line drawing of the face (just use an hB) and have a crack at making it more 3D with shading with darker graphites and maybe even charcoal(for the very dark areas). After this exercise in light and shade, you will appreciate the benefits of a live model or a few good photos, even if you are not going for a portraiture-type likeness, the subtle nuances of lighting are much easier achieved with strong contrast in lighting (in your reference or on your model).

Here, I have used hard black charcoal pencil for the darkest tones of the hair, 6B and 2B for the skin shades, a blending stump,  and a Derwent battery-operated eraser for all of the little tiny highlights. I imagined that the light was coming slightly from the left / front of the model and shaded accordingly. Make sure you don’t try to lay down a lot of graphite then try and use charcoal over it to darken (graphite forms a metallic finish which won’t receive the charcoal at all well.

It is handy to have a gum eraser, a kneadable eraser(for the soft mopping out on cheeks etc) and a battery-operated as well (for the tiny highlights).