1. When the model sits opposite the observer, the eyes are horizontal in the observer's field of vision, that is, parallel to the horizon-the same mouth;

2. If the model is at a certain angle with the observer, then the vanishing point of the perspective of the eye is on the horizontal line of the observer's eye-the vanishing point of the same mouth.

3. The nose is vertical, that is to say, the vertical relationship with the visual horizon, pay attention not only to the vertical with the visual horizon, but also to the vertical curvature of the head, otherwise it is a flat face.

4. Drawing pays attention to the whole, and any part that is too prominent will destroy the overall sense of the overall picture, so drawing should have a clear relationship and not jump too much.

I hope I can help you, come on!