Lie face up, with your back flat on an exercise mat, your legs extended out in front of you and locked together and your arms by your sides. This is your start position. Proceed to lift your legs from the floor. Keeping them extended and locked together until your legs are at a 90 degree angle to your torso. Hold for a moment. Then lower back down slowly to the start position. Repeat for the required number of reps (or time).