- Don't sketch!
Draw each line once. You have to get it right the first time.
If it's wrong, don't try to fix it. Just draw it again.
- There are two separate areas that you'll be improving at the same time with this exercise: The quality of your line, and the accuracy of your observation.
You need to draw each line once. You need to draw it in the right place. And it needs to go from thick to thin to describe forms and shadows.
You need to be able to instantly gauge the size, shape, and distance of the features to each other, and the size and shape of the head relative to the features.