I start the Underpainting with a Wash In of Raw Umber mixed with Medium (2 drops 50/50 Medium. 1 drop Clove Oil). Since I live in California, where it gets pretty warm, the Clove Oil prevents the fast drying brown pigments from drying out too fast.
The idea is to cover the canvas with enough Raw Umber to be able to achieve a nice gradation from the dark shadow values of the models face to the lightest lights. By wiping out the lights with a cotton rag, it makes it easier to model the face. I like to allow for a full days of work, without too many interruptions, so I can model as much as possible before the paint dries (drying time ca. 24 hrs).
Somewhere, someone once said that at each stage the painting should be able to stand on it's own. I try to aim for that, but at the same token keep in mind that a painting is a series of corrections, wereby the artist should aim to get closer to the optimum result with each successive layer. At this stage what is important is to get the bone structure of the head, figure out what the shadows and the light are doing and just cover the canvas with paint.