Sit on an incline bench with about a 45° angle with dumbbells in each arm and arms fully extended at the side. Keep your palms facing in towards the bench. Make sure you keep your upper arms as still as possible while you curl the dumbbells straight up. Your wrists should remain locked in position with the palms facing in towards each other. Remember, this is not your standard dumbbell curl where you actually want to supinate the wrists extensively—keep your palms locked in the hammer position here.
This movement can be done by alternating arms—as with standing hammer curls—but with the seated, incline version I prefer to work both arms simultaneously in a nice, steady rhythm.