Lie on your back, fully extend the legs. your entire body should be parallel to the ground. Place arms above the head or off to the side for added stability. Slowly bring the left leg toward the chest, bending at the knee as it comes to the chest. Bring it as close as it will go.
When the left leg is at the chest, bring the right leg, bending at the knee, toward the chest while at the same time, extending the left leg back to its straight position. (Note: To put more emphasis on the obliques, one may bring the torso up (with the hands behind the head), and twist the torso so one's elbow meets the opposite knee as it comes towards the chest.)
Do not allow the legs to touch the ground at the bottom (keep them 2-6 inches above the ground). Hold the lowered position for 1-2 seconds before alternating to the other leg.