To find the values in the human face, all we need to understand is the structure of the skull…
Now as you draw look for all the places on your face where the bone is directly under your skin, those are the lightest parts!
Where there is no bone directly under your skin (under your cheeks, eye sockets, between your eyes) there you will have all the darkest shadows!
Look how all the shadowed areas are in the places where your skull does not stick out.
Twenty-First Century Art and Design 