STARTING POSITION:
Select the appropriate resistance in the weight stack. Kneel on the floor so that the pulley is behind you. There should be a cable with a handle that is attached to a high pulley. Grab it. Contract your pelvic floor and core while keeping your chest up. Retracting your shoulder blades, pull the handle towards the shoulder so the elbow is around 90º and de hand is at the height of your head. Your arm should be in line with your back, your elbow should be around your shoulder height and the palm should face forward.
MOVEMENT:
Do an internal rotation of the shoulder avoiding any other movement of your body so that your palm is facing the floor. Exhale during the movement. Return to the starting position with a smooth movement, preventing moving weights from touching the rest of the stack. Repeat the movement for the specified amount of repetitions and then repeat with the other arm.
TIPS: The exercise key is quality over quantity. The trick is to perform the exercise using proper form.