1. Measure your underbust/ribcage.
2. Measure your overbust.
3. Subtract these measurements to calculate your cup size.
Keep in mind that sizing runs differently between countries and companies, especially once you get above a D cup.
Now that you've got starting point, you begin the process of finding the ideal fit for your body and the type of bra. So, how to do this?
1. Do some initial research. Read bra blogs. Join Bratabase and ask lots of questions. Get to know various bra styles and terminology. Poke around on the internet, ask fellow busty friends. In conjunction with step 2, this should give you an excellent foundation upon which to find a well fitted bra.
2. Stare at your boobs in a mirror and figure them out. Are they spaced wide or close? Do they droop a lot? Do they sit low or high? Is the majority of your breast tissue carried in the top or bottom of your breast? Do your boobs have a particular shape you wish to highlight or hide? Is your breast tissue easily moldable or not? Are they different sizes? Do they fluctuate in size often?
Breast shape, size, etc. greatly influence the styles and shapes you want to seek out and wear. By knowing not only your bra size, but your breast tissue characteristics, you can better serve yourself with step 3, finding a bra.
3. Find a bra that fits you as perfectly as possible - one that encases all your breast tissue, holds you snugly around the ribcage, gives most of it support through the band, doesn't have spillage or quadraboob issues, and that you feel you can move around in comfortably. It is important to keep in mind that a well fitted bra may feel much tighter in the band than you are used to, but give it a chance. For some women, this step is easy, for others it may take a few bras before finding the right fit.
Assuming you measured everything well initially, this bra will be close in size to the measurement you made initially. It may be a band size up or down, or a cup size up or down.
4. Do more research. Read more bra blogs. Join Bratabase and ask lots of questions. How do different brands compare? Some brands are notorious for running small in the cup, or tight in the band. Others have very generous sizing. Armed with the knowledge of a well-fitted bra in a particular brand gives you the knowledge you need to buy a bra in nearly any brand, as long as you realize no two bras are built or fit the same.