The practice of sketching is the practice of seeing. When we draw we are tempted to draw what our mind thinks a thing should look like, not how it actually looks. To counteract this tendency we follow a process of study and sketching.
One-minute memory drawing
Look at the subject for one minute. Study as much detail as you can. Then draw for one minute without looking at the subject.
One minute blind contour drawing
Look at the subject for one minute, drawing the subject without looking at the paper–blind, so to speak.
30 second gesture sketch
Draw the subject for 30 seconds, capturing as much detail as you can.
15 second gesture sketch
Draw the subject for 15 seconds, capturing as much detail as you can.
Five second gesture sketch
Draw the subject for five seconds, capturing as much detail as you can.