Here is a great Bra Fitting Guide by Curvy Kate featuring their model Laura Butler.
I totally agree with all these steps. There are too many people on the internet with too many wrong opinions on how to fit a bra but Curvy Kate have got it right.
Remember these Top Bra Fitting Tips:
Tighten up your straps so that two fingers can fit underneath. Only 20% of the support should come from the straps.
Always start with the back band on the loosest hook. You can tighten it with wash and wear over the months that follow.
Lean forward and give your boobs a jiggle holding the point where your straps meet the cup.
Always check your wires too! Right from your underarm, all the way around and in between your boobs the under wire needs to sit flat. Give all these tips a go and notice the difference it makes to your boobs, your posture and even your outfits!
More more info check out Curvy Kate's Blog on Tips for the Perfect Fit.