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). 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.
Lift your bottom leg up until it touches the bottom of the bench, squeezing your groin muscles.
Breathe in through your nose and out through your mouth, focusing on feeling your abs and obliques.