For reference, I'm putting up a list of links to my posts on proper bra fitting. If you're new here, I highly suggest you go through them. I hope they are helpful!
How to measure your bra size, the correct way!
Why you should go get a professional bra fitting
How to Put a Bra on Correctly (It's Important!)
Signs of a Bad Bra Fitter
Signs of a Poor Fitting Bra
Why do so many bra measurement guides tell you the wrong way to measure?
Change your bra, NOT your body!
Reader Question: Bra-fitting help/underarm "fat"