Lay on your side with your feet just underneath the bench. Place your top foot on top of the bench, supporting your weight on your instep (make sure the bench is padded, or place an additional pad under your foot). Your other feet on the ground so it will help to hold your body up. Place your bottom elbow and forearm directly under your armpit, and squeeze your fist tight.
Lift your hips off the ground by driving your top foot down into the bench — you should feel the groin muscles of your top leg working hard.
Keep bottom leg down so it helps you on holding your body up.
Breathe in through your nose and out through your mouth, focusing on feeling your abs and obliques.