Hold the dumbbell close to your chest, and lie on the bench (perpendicular to the longer edge), in such a way that, only your upper back and your neck are on the bench. Now, stretch out your legs, then bend your knees, and keep your feet flat on the floor. Lower your hips a little. Now, grip the dumbbell firmly, and raise it over your chest (see that there is just a minor bend in your elbows when you do so).