This drill is important to practice for a variety of reasons. First off, it helps with your preparation for the pirouette; where your arms and your lower body are placed. The preparation helps the power of your pirouette. This drill also practices the correct posture and correct weight placement. It helps to learn how to stay on top of your standing leg, to lessen your chances of falling out of the pirouette and to maintain the correct posture and weight distribution.
On another note, even though it is more intended to help your technique during your pirouette, it will also help build your range of motion of your hip joint, and will help strengthen the muscles in your feet, calves and gluteus maximus.
1- Stance- This is how you start the pirouette. External rotation from the hip, (if this is a right turn) right leg behind the left. Your left arm is extended to the side, and your right arm is positioned in a half circle anteriorly.
2-Preparation- Bend your legs 45 degrees and be ready to shoot your right leg into "passe" to your left knee as fast as possible. Your right foot is in plantar flexion as soon as it leaves the ground. Your left leg is in a "relevee" /or on the ball of your foot. Be sure to keep hips externally rotated, and hips squared off. Your leg should never come into medial rotation because ballet dancers are always turned out. Your left arm comes in to meet your right completing that circle you will make with your arms.
3- Follow Through- Hold that position for a split second (as if you were actually turning and completing the pirouette). Be sure to keep the standing leg straight, and abdominals tight. Keep your body still, and tall; as if a string was pulling you from the ceiling.
4-Landing- Although you are not turning, coming down from this position helps your landing skills. Gently roll down on the left leg from the ball of your foot to your heal. Eversion and inversion of your feet can lead to injury, so be careful to stay in the middle of your foot as you land. As roll down to your heel, take the right leg and shoot it down behind the left leg, to the position that you started the pirouette. Extend the left arm, and keep the right arm in the same position, in that half circle.
Repeat as many times as needed. You can even do a few of these drills, a few times in a row, and then go straight into a pirouette. It will help you get into the right mind set of how exactly your body should be moving and feeling in the pirouette.