Lie back on a bench holding a dumbbell with both hands. Extend the elbows just keeping them slightly soft so that the dumbbell comes above your chest.
Keep the feet firmly planted to the floor and engage the glutes
Make sure that you press the shoulder blades against the bench.
Breath in and move the arms backwards over the head without any movement at the elbows till you feel a good tension on your chest and muscles under your arms.
From here come back to the starting.
Breath in to repeat the movement.