Choose the appropriate dumbbell weight. Sit on the bench with your torso firmed up and pelvis in a neutral position and legs shoulder-width apart. Grab the dumbbell and move it upwards while holding it with both hands. Your elbows are at your ears, wrists are firm and not bending.
From the starting position, while not moving your elbows forward or sideways, execute the movement by bending your elbows. The dumbbell moves behind your head until your elbows make about 90 degrees angle. Keep your wrists firm and not bent throughout the entire movement. Inhale as your hands are moving down behind your head and exhale as they are going back up to a full stretch.
Make sure your torso is upright. You only bend very slightly forward and avoid over bending at the waist. Keep your pelvis in a neutral position. Keep your elbows fixed and avoid excessive motion forward or sideways. Avoid involving the trapezius muscles (lifting your arms) when raising the dumbbell.
Begin with lighter weight to master the technique well. If possible, exercise in front of a mirror to watch the correct position of your arms, elbows and knees.