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Your forearms should be flat against the ground and your shins should be nearly flat against the ground, as well. Keep your head, neck, and back in a natural position. They should be straight without being rigid, and your back should angle down slightly toward your front half. You should try to raise the thigh to about 45 degrees. Your knee should be bent, and your heel should face upward.  Keep your back straight as you lift you leg. Hold yourself in this position for five seconds. Slowly lower your left leg to your starting position. Once lowered, lift the left leg again in the same manner. This exercise should be repeated at least five times during a routine with one leg. When you have finished exercising your left leg, repeat the same steps using your right leg. You should lift your right leg the same number of times you lifted your left leg.

Summary:
Rest on your elbows and knees. Raise your left thigh slowly. Lower the leg and repeat. Repeat, switching legs.