Photoshoped to get the tones.
1. Shade here to get the depth of the eyes. Remember this parts is deep-set
2. Shade the lower part of the nose to get deepth. Remember this part is closer to the observant.
3. Emphasize the nasal holes
4. Nose casting shade. It's sharper in the border. I don't use to make it blurred. It's darker. It gives variety to the execution of the shades.
5. Emphasize the corner lips. That part is small but deep.
6. In the middle of the upper lip it creates a special volume.
7. The volume of the bottom lip is very important. Even it casts a shade but not that marked as the nose. In this case I didn't cast it.
8. It's not necessary but you can get some of deepth if you make some strokes for the ocular cavity.
9. Shading the volume of the lower eyelid contributes to the simplification of the shading and gets deepth to the eyes.
10. This is a synthesis. A few strokes can give an idea of a complete part. It 's not necessary to outline all.