Contract your Abs and lift your legs and upper back off the ground; at this point, only your lower back and upper buttocks should be in contact with the ground (your arms can also lightly rest on the ground).
Lift one leg upward and let the other one go down to just above the ground.
Switch their positions and keep alternating.
Form tips
There should be no space between your lower back and the ground.
Do not crane your neck forward: keep your neck neutral and look to the ceiling.