Lay the dress shirt flat on a smooth surface
Button the cuffs and straighten the sleeves
Find the center back of the shirt at the collar seam
Place the end of a measuring tape at the center back neck
Measure across the shoulder to the shoulder seam
Continue down the outside of the sleeve to the end of the cuff
Record the measurement in inches or centimeters
Repeat on the other sleeve to confirm consistency
Use the longer measurement if the sleeves differ slightly