First thing first, I know that some find princess seams a pain to sew. Having sewn quite a lot of princess seams (my wedding dress had such construction), I have found that the a smooth finish can be achieved by following this tutorial.
Now this little tip is out of the way, let's get started with the tutorial.
I say that princess seams are super easy to adjust, because all you need to do is to "shave off" some of the excess curve. Normally I take the piece with the outside curve, i.e. the side front (I'm using the By Hand London Elisalex pattern here), and simply smooth out the curve around the bust area.
I'm demonstrating this here using the pink highlighter:
A close up:
I use the French curve for this, but this can also be achieved free hand.
The next step is to cut off the excess following the pink line, and then you are good to go!
I should add the usual caveat now that as with the other 2 parts, this is simply what I do to adjust for a smaller than B cup bust, rather than the "right" way. It works for me, and I hope it does work for you too :)
As always, any questions please ask, and I'll do my best to answer.