Lie on the ground, facing up, with a sturdy object near your head, which you grip with both hands.
Tuck one leg close to your chest, while keeping the other one fully extended and in line with your torso.
Engage your Abs , pull on your hands and hinge at the shoulders to lift your whole body straight up, until your hips are above your shoulders (or nearly).
Then lower yourself back down with control and repeat.
The primary muscles work equally regardless of which leg you extend, so it is not necessary to count the exercise as unilateral: it is enough to alternate the legs with each set or repetition.
Form tips
Keep your straight leg aligned with your torso: do not bend at the hips (that would decrease the stimulus).