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).
Move each leg to the inside, choosing one leg to move on top of the other one.
Open them back and repeat while switching which leg goes moves on top.
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.