This is an exercise. Like contour drawing or shading an egg. The scribble drawing frees you of detail leaving you to concentrate on proportion and average value. The value is the amount of light that a portion of a painting or drawing reflects. In the scribble drawing, the values are linked to the density of marks made on the page. There is very little detail.


You need to do it quickly, and without reference. In the image below, this was only a two minute scribble. Squint while you do it. This makes the values appear to your eye-brain system. As you make a mark, it will add to the value-system, and to proportion. If the proportion needs adjusting, then move towards the corrected area. If it looks right, then concentrate on that a little more. Keep in mind, where is the light source. In this drawing, the light is directly overhead which is why the upper cheekbones are lightest.

It's not meant to be a finished artwork. Let your hand move rapidly, with movement coming from the shoulder and elbow. Let your mind be free of fear of failure. If you like what you see, then later, it is possible to re-draw this into a full rendering. 

This is a new category. I'll be listing and describing some drawing exercises. After all, if you are a musician, then you practice scales or rifts or something similar. If an athlete, then squats, lunges and other strength exercises help performance. But even thinking about your sport can improve performance. This surprising snippet of information is backed by research as demonstrated in an article stating "Mental practice-based rehabilitation training to improve arm function and daily activity performance in stroke patients."

So this implies that you can get better at drawing just by thinking about drawing. If you are caught in a queue, or on a bus with nothing to do, then close your eyes, and draw or paint a picture in your mind. Choose and mix the colors, arrange the composition, build up an image, make corrections. When you next put pencil to paper, or paint on canvas, you just might find a little new skill or two.