Here's a quick 1 minute demonstration I did for my class. It's a combination of gesture and contour drawing with a Micron pen. I scribble from the inside of the shape to the outside, adjusting as I go for the proportion and angles that I need.
The whole idea is to use your sketchbook all the time, not worrying about how to perfect what you're doing......just do it! Practice creates freedom, freedom and enthusiasm create artistic growth.