Choose the appropriate dumbbell weight taking account of your fitness level and training type. Set the bench for upright sitting. When you are sitting in the middle of the bench reach down and grab the dumbbells with an overhand grip and lift them to bring your hands to shoulder level, bent at the elbows and without rotation in your wrists.
From the starting position push both dumbbells upwards in parallel above your head, extending your elbows but not rotating your wrists. Return to the starting position the same way. Inhale while at the lower (starting) position and exhale while going upwards.
Choose the appropriate dumbbell weight taking account of your abilities, overall fitness level and training type. Start using lighter weight to master the technique well. Return to the starting position with a slow, controlled motion rather than quickly relaxing the tension in your muscles. If possible, exercise in front of a mirror and avoid using the trapezius muscles. Fix you torso while doing the lifting to avoid bending at your lower back.
Exercise in front of a mirror to make sure your arms are in a symmetrical position and the entire motion is synchronized. It is better to exercise with assistance of a partner or trainer when using heavier weights.