Tip:if you don't have a measuring tape, you can use a ruler and a piece of string
Start with a collared shirt that fits you well. Lay the fully buttoned collar flat. Measure with tape inside of the collar going around to the far end of the opposite buttonhole until tape overlaps.
Measure just under arms and across shoulder blades holding tape firm and parallel to the floor.
With arm relaxed at your side and slightly bent, measure from center back neck, over the shoulder, down the outside of the arm to the wrist. Sleeve length is always in whole numbers so round up if necessary.