Start standing in front of a sturdy box, or something else resembling a box. Start with a low box with these and work up in height as you learn your abilities. Crouch down and explode upward, swinging your arms for momentum, and landing on top of the box. You want to land lightly and with your knees bent, so as to lessen the load on your knees and the impact on your body as much as possible. Stand up and jump back off the box backward with both feet at the same time. Repeat.
Make this easier by lowering the height of the box and/or stepping off the box one foot at a time.
Make this harder by raising the height of the box and/or going faster.
This video is used in a rock climbing training program at http://www.trainingbeta.com.