2 Create a silhouette
Underneath my lines I opaquely fill in the silhouette of the head with a light grey tone, then merge the lines down so I have a single opaque shape. This will help speed things up if I want to alter the background later on in the painting process. Using a mixture of Darken and Multiply layers I start to suggest the direction of the light. I also frequently use the Smudge tool, set to a Blender Brush, to push shapes around easily.