If you can draw a line, you can draw ANYTHING. The artistic skill here is not whether or not you can draw a line, it is learning to draw the lines you really see, rather than the ones you THINK you see. Let's train your right brain to observe the world before you draw any lines.
This technique comes from Betty Edwards book Drawing on the Right Side of Your Brain.
Edwards philosophy is that we view the world in two ways. When we are viewing it with our ‘left brain’ we see in symbols of how we think things ‘ought’ to look like which hinders us drawing things as they really are. On drawing upside down Edwards notes:
"For reasons that are still unclear, the verbal system immediately rejects the task of “reading” and naming upside-down images. L-mode seems to say, in effect, “I don’t do upside down. It’s too hard to names things seen this way, and, besides, the world isn’t upside down. Why should I bother with such stuff?”
Well, that’s just what we want! On the other hand, the visual system seems not to care. Right side up, upside down, it’s all interesting, perhaps even more interesting upside down because R-mode [Right Brain] is free of interference from its verbal partner, which is often in a “rush to judgement” or, at least, a rush to recognize and name."
She talks about how her student’s drawings would instantly improve when drawing this picture upside down. Suddenly difficult areas like the crossing of the legs would look extremely accurate where right-side-up the foreshortening would often render the copy a very poor likeness indeed.
Start with a contour drawing (the outline of the subject). Keep the drawing upside down to help minimize the voice of the dominant left side of the brain that would like to label. Draw only the lines you see as you see them. Use your pencil to measure distance between lines, check angles, etc. Both the image and the drawing are upside down.