If you can, start with a photograph and trace the main features.
Basically, if you don't have a photo, heads and faces have a strong similarity, so start with an oval, then refine it to get the base shape. Add features: start with the basic shapes (cone for the nose, slash for the mouth, circles for the eyes, hairline, etc.) Then refine the features to what the face actually looks like. Make sure they are in proportion to the reality.