The ability to break human features down to their basic components. When we draw the eye, we start with an oval shape like the process below.
Sketch lightly with your pencil, blocking in the basic shapes and lines. Once your proportions are pretty much correct, then squint at the image thorugh your eyelashes and draw in the shadows and midtones. To simulate skin, smudge the pencil or charcoal with your smudge stick or fingers.