Lay flat on the ground facing up. Engage your core and keep your upper back off the floor.
Sligthly lift your straight legs so that only your glutes and lower back are in contact with the ground.
From that position, crunch up to form a "ball" with your body, surrounding your tucked legs with your arms.
Slowly lay back down without losing engagement to connect consecutive reps until the goal is reached.
Progressions And Regressions