Measure from the top of one shoulder to the other, along the upper portion of your back. You will need to have someone's help with this. For best results, enlist a friend's assistance to ensure this measurement is as accurate as possible!
Take all measurements wearing your usual undershirt.
Wrap the measuring tape comfortably around the fullest part of your chest. The tape measure should lay flat against your body without being pulled tight or causing an indent.
Using your shoulder joint as a reference, measure from the joint of your shoulder down to your elbow, then from your elbow to your wrist.