Can't say I have direct experience, but I think you measure the boobs from the fullest part and get a number (example: 36), and then you measure underneath the boob to get the torso size (ex: 32). Subtract the torso size from the "full" size and that gives you your cup size. 1= A, 2 = B, 3= C, 4 = D, etc.

The number in the bra size is the torso size, so that person would be a 32D.

Umm, I'm not sure what a double letter like "DD" would mean though...