A: MEASURE YOUR UPPERBUST
Measure around the fullest part of your bust (usually over the nipple). This is best done while you're wearing a bra. Hold the tape gently so that the tape is resting lightly on your body. Make sure that the tape measure is straight across your back. This measurement is your UPPERBUST.
B: MEASURE YOUR UNDERBUST
Stand with your arms resting by your side and ideally get measure directly below the bust. Make sure the tape is right under your breasts, is straight all the way around and is as tight as possible. This is how the underband of the bra will fit, so the tape should be tight, but not uncomfortable. It's best to do measurement while you're not wearing a bra. Just be careful not to sit the tape over your bra or underwire. This measurement is your UNDERBUST.