Lay on your back, with your hands anchored behind your head.
Engage your lats & core by actively pulling with your arms, elbows forwards.
Raise your legs off the floor - Keep them as low as you can while maintaining Core tension - lower back should remain in contact with the floor.
Generate tension throughout the whole body, & flutter your legs for the duration of the hold. Point your toes, squeeze your glutes & keep the lower back in contact with the floor, core braced.
Don’t hold your breath.