Chest
Take a soft tape measure and wrap it under your armpits at the widest part of your chest. The tape measure should be snug around your chest and shoulder blades, but not so tight that it compresses your chest.
Shoulder
With the shirt laid flat, back facing up, place the tape at the point on the bottom seam of the yoke where the yoke attaches to the sleeve. Measure straight across to the same point on the other side of the shirt
Sleeve
Place one end of the measuring tape on the bottom edge of the sleeve, an inch from the armpit. Measure up to the top of the sleeve such that the measuring tape is perpendicular to the top folded edge of the sleeve. Do not stretch the sleeve but be sure it is spread out flat.