1) Stand straight against a wall with your back and buttocks pressed against the back wall; your feet should be together.
2) Reach out which ever arm is against the wall straight in front of you, keep palms facing downward toward the floor.
3) If you have someone able to help, have them measure from the wall to the tip of your outstretched thumb while your arm is reaching out in front of you. If you are on your own, use care in measuring from the wall to the tip of your outstretched thumb.