Arms relaxed at your sides, measure around the fullest part of your chest/bust (around shoulder blades and under arms). Do not pull tight; the measuring tape should be straight and flat against the body, yet comfortably loose.
Length
Stand up straight. Either use a mirror or have a friend measure from the top of the shoulder about an inch from the neck. Extend the measuring tape down along the body to the distance shown on your selected size. This will be the approximate position of the bottom edge of the garment.