Wear a well-fitting dress shirt or stand naturally with your arm relaxed at your side
Start the tape at the center back of your neck, just below the collar
Run the tape over the top of your shoulder and down the outside of your arm
Bend your elbow slightly and measure to the wrist bone where the cuff should end
Keep the tape snug but not tight
Round to the nearest half inch or centimeter
If possible, have someone else take the measurement for better accuracy