Simplified planes of the head.
For an object to be 3 dimensional it has to have width, height and depth. Once an object is built it will have planes that will define its form.
Using a sphere to build the wire frame of a Manikin head, we establish planes that define the front, sides, back, top and bottom views of the manikin head. By memorizing these planes we can figure out how to apply the light and shadow values.
Apply the basic values first, one at a time. 1. Light, 2. Shadow ( note that you can start with either the light or the shadow, this is the artist choice ) and 3. Core shadow/reflected light( note the core shadow is a by product that results from reflected or bounce light ). Always be aware of the clock, Manage your time and work fast. If you have time left go back and apply the bling elements, Highlights and Cast Shadows. It's gonna look awesome.