Step 1: Step 2:
wrap measuring tape just below your bust; the tape should fit snugly. make sure you are wearing a bra with little to no padding.
If you don't have a tape measure, use a long piece of ribbon, then lay it against a flat ruler to find your measurement
measure around the fullest part of your bust. the tape should fit comfortably and slightly loose. round up to the nearest inch or centimeter.
Don't pull too tight on the tape, instead let it rest over the fullest part of your bust.