Step by Step Guide
Begin lying on your back with the legs straight and hip distance apart. Arms can be done at your side with the palms down. Engage your abdominals by pulling your belly button toward your spine - hold that.
Raise both legs off the ground a few inches while keeping them straight and while maintaining that neutral position with your spine (by engaging the abs).
Lift just one leg straight up toward the ceiling while keeping it straight. The other leg can stay hovering above the ground.
Now switch the legs: lower one down to the hover and raise one up to the ceiling.
Continuing alternating or scissoring the legs, all while keeping the abs engaged.
Repeat for the desired repetitions.
One common mistake is losing abdominal control which is exhibited by the lower back lifting off the ground into an extended position. To avoid this you can go into more of a posterior pelvic tilt to exaggerate avoiding the anterior tilt and/or lumbar hyperextension position.