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...