It is best to have another person present, who would do this measurement for you. Stand straight and make sure your arm is bent slightly, then place the leading edge of the measuring tape at the center of the back of the neck, where the collar touches the shirt. Run the measuring tape over your shoulder, down the back of the arm passing over your elbow, to a 1/2" below your wrist bone. Do not adjust for shrinkage. We will do that for you.
If you do not have anyone to help with body measurements, take one of your dress shirts that fits you perfectly, button it up and spread it out face down on a flat surface. Place the leading end of the measuring tape at the center of the back, just below the collar band. Run the measuring tape along the top of the shoulder and down the sleeve to the end of the cuff. Make sure the sleeve firmly away from the dress shirt body as in the diagram below. Do not stretch the shirt sleeve, but be sure it is fully extended. Do not add extra to account for shrinkage. We will do that for you.