Lying on your back, place the ball at the back of the shoulder joint - in the soft tissue / tight area just below the back ridge of the shoulder blade but don't roll over the boney part, stay in the muscle part. then slowly roll over onto the ball to apply the pressure. Once you have enough pressure and found the tight spot you can slowly move the arm up and down on the floor in an arc to add in more mobilisation and release through range

Be careful not to apply too much pressure as this can be a sensitive area. Aim for 1min reps, doing 2 or 3 repetitions in a session.