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Begin standing upright with your feet hip width apart.  Dip into a quarter squat position by bending at the knees and hips while pushing your arms behind your body.  From this position rapidly accelerate your body upward by extending your hips, knees, and ankles and aggressively swinging your arms upward.  Using maximal effort, jump as high as you can vertically and pull your knees up toward your chest.  When you return to the ground, absorb the landing with your hips and knees.  Reset in the upright position and repeat for the designated number of repetitions.  This drill can be performed as individual jumps or repetitively.  When performing repetitive jumps, as soon as you return to the ground jump again.

A good tool you can use to be sure you are jumping vertically and not floating forward or backward is to start with the balls of your feet on a line and check your position when you land.  You should land in the same spot you jumped from.