How to do it
You need a bench or a sturdy thigh-high platform to lean on when doing the exercise, so secure that first and place a dumbbell on the floor to one side of it. Put your left leg on the bench and grab the far side with your left hand, then bend over so your upper body is parallel with the ground. Reach down and pick up the dumbbell in your right hand with a neutral grip (palm facing you), then hold it with your arm extended, keeping your back straight.
Bring the dumbbell up to your chest, concentrating on lifting it with your back and shoulder muscles rather than your arms. Keep your chest still as you lift. At the top of the movement, squeeze your shoulder and back muscles. Lower the dumbbell slowly until your arm is fully extended again. Do all your reps on one arm before switching to the other side.