Lay a well-fitting shirt flat on a smooth surface
Smooth out wrinkles and align the shoulder seam
Start at the center back of the neck or shoulder seam, depending on the shirt style
Measure along the top of the shoulder to the shoulder seam
Continue down the outside of the sleeve to the end of the cuff
Keep the measuring tape straight and follow the natural curve of the sleeve
Record the measurement in inches or centimeters
For best accuracy, compare with a shirt that fits well
Repeat the measurement on the other sleeve if needed