Sit or stand up straight.
Bend your elbow on the arm to be exercised, and raise your arm up to the side of your head.
Using your other arm, lift your forearm on the side to be exercised up towards the ceiling, straightening the elbow.
Your arm should remain by the side of your head.
Take away the support of your other arm, and slowly bend your elbow again, bringing your arm down towards your upper back.
Your upper arm should remain by the side of your head.
Repeat this movement, ensuring your straighten the arm to be exercised with the effort of the other arm, and lower on its own.