Lay the shirt flat on a smooth surface
Smooth out wrinkles and align the sleeves
Find the center back of the shirt collar
Measure from the center back of the neck to the shoulder seam
Continue measuring from the shoulder seam to the end of the cuff
Add both measurements together for total sleeve length
If the shirt has raglan sleeves, measure from the center back of the neck to the cuff in one continuous line
Compare the measurement to the shirt size chart if needed