Lie on your back and engage your Abs to lift your legs, butt and shoulders off the ground ( Dead bug position), so that only your lower back is in contact with the ground.
Without moving anything else, hinge at one hip to lower that foot to the ground.
Return that leg to the starting position and repeat on the other side.
Form tips
Keep your head neutral: do not crane your neck forward. It helps to look at the ceiling.
Aim to be in contact with the ground with as small of an area of your lower back as possible. This will create additional Abs engagement.