Choose an appropriate dumbbell weight depending on the type of training and your level of fitness. Set your bench to an upright position.
Sit on the exercise bench and lift your arms by flexing them at the elbows while holding the two dumbbells in front of you at about upper chest level with your palms facing your shoulders. Your arms should be next to your torso.
Continue lifting both dumbbells at the same time above your head by extending your elbows and rotating both wrists so that the palms face forward in the top position. Go back to the starting position in the same way while breathing in in the lower (starting) position and breathing out as you go up.
Choose the dumbbell weight to match your abilities, overall fitness and type of training process. Start with lighter weight to master the technique well.
Avoid fast and uncontrolled motion as you are coming back to the starting position resulting from relaxation of the muscle contraction.
Watch yourself in the mirror to make sure you eliminate use of the trapezius muscles.
As you breathe in and out, make sure you keep your torso straight and eliminate overarching your lower back.
Check for symmetric position of your shoulders and synchronization of the whole motion by looking in the mirror.