This method of study was introduced to me by my awesome buddy Anthony Jones, aka Robotpencil.
Step 1: Using no reference draw or paint something
Step 2: Do a photo study,
Step 3: Try to apply what you learned without using reference.
Top 3 Things I learned:
1. Eyelashes make up the biggest and darkest values.
2. The bridge of the nose can be left almost white and given shape by the shadows behind it.
3. Lip Values can be more of a flatter gray and contain wrinkles that travel vertically.
A soft light gray value added to the cheeks can give a more youthful appearance. incorporating the tearduct into the eyes, the jaw outline of the face is one dominant curve but adding 2 slight indents can imply muscles underneath the skin. adding grey values on the bottom lid, closer to the tearduct can push the round form of the skin around the eye. At the outline for the forehead has a slight indent just above the farther eyebrow.
NOTE: These notes are about the things I learned from this specific photo reference. They might not apply to all women's faces but can help to provide a general understanding of values.