Stand and place a towel or a strap over one shoulder so that the other end hangs low behind your back.
Grasp the other end behind your back using the hand you want to stretch.
It can be it easier to grasp if it is longer and if you bend sideways towards the same side while reaching for it.
Then using your other arm pull the towel or strap upwards so that the arm behind your back moves towards your waist and upper back.
Keep your posture upright and chest wide.
Then in a controlled manner return to the starting position and relax.
