Using a soft tape-measure1, firstly measure around your chest under your bust, at the level where the lowest part of your bra should sit. Try not to let the tape slip down at the back; it needs to be level all the way along. Take this measurement (in inches): if it is even, add four, if it is odd, add five. The resulting number is your band size.
Next, measure around the fullest part of your breasts; don’t squish them unless you want your bra to squish them too2. This measurement in inches is your bust size.
To obtain your cup size, subtract your band size from your bust size. If the two are the same, you are an A cup. If your bust size is one inch larger than your band size, you are a B cup, two inches larger and you are a C cup. This continues through the alphabet, though not in a strictly linear fashion, due to the presence of cup sizes such as DD.
Hope that helps. xxx