Identify the light source
Mark the lightest areas on the face
Shade the shadow side of the face
Darken the eye sockets, nostrils, and under the nose
Add shadow under the lower lip and chin
Blend the shading smoothly
Keep highlights on the forehead, nose bridge, cheekbones, and chin
Build shadows gradually in layers
Use softer shading on rounded areas
Use sharper contrast near facial features
Refine edges between light and shadow
Adjust values to match the face structure