Lie on your back on the floor with your arms extended behind your head holding onto a grip medicine ball. Have your legs on the floor.
With your knees slightly bent and your legs together raise your legs upwards contracting your lower abs and lifting your butt off the floor.
Then lower your legs down again, tapping your feet gently on the floor.
Then repeat the exercise.
If you don’t have a grip medicine ball, you can hold onto something else that’s stable. I’m using a 10kg grip medicine ball.
The more advanced version of this exercise is to keep your legs straight and not touch the floor with your feet inbetween reps.