Please note that all measurements should be taken whilst unclothed.
To establish a correct bra size: Take a tape measure, hold it straight and firmly measure around your rib cage, across your back and just beneath your bust. If the measurement is even add 4 (four) inches to it. If the measurement is odd, add 5 (five) inches. You now have your bra size.
To establish a correct cup size: Measure yourself as before, but this time across the fullest part of your bust. Hold the tape firmly, but not so tight that it flattens you. If there is no difference between this measurement and your bra size, then you are an A cup. If the measurement is 1" (one inch) greater you are a B cup. 2" (two inches) greater you are a C cup. 3" (three inches) greater you are a D cup. 4" (four inches) greater you are a DD cup.