Painting the Monochrome...
The portrait begins with a monochrome (one-color) underpainting. This is also called the imprimatura.
First: The canvas is first prepared with gesso, sizing, and white lead. When this is dry, the subject is sketched with paynes gray thinned with turpentine. The sketch is always done with paint thinned to an almost "watery" consistency. The goal is to place the subject on the canvas in the correct proportions.
Next: Still using paynes gray and working from a photo, the lights and darks of the face are "blocked in." The goal is to establish at least 3 gradations of tones - highlights, midtones, and darks. At least three tones are needed in order to achieve a 3-d look.
Next: This is an excellent example of what the monochrome underpainting should look like. It should definitely retain a "sketchy" quality at this stage. Especially avoid blending at this stage. The paint is allowed to dry before proceding to the next stage.