think about the mechanics of breathing, and study other things that have already done it!
Mechanically, it's an expansion of the upper chest and a tensing of the gut, as seen below:
And other mediums that have used breathing motions for more interesting idle animations are numerous! For example, here's Balrog, from street fighter:
Here, instead of emphasising chest expansion and the breathing itself, they instead used it to create a posture change that gave him a more guarded, trained look. They minimized it in favor of a boxing bob!
And here's blanka, with emphasis on his chest and shoulder movements to create a more bestial, almost gorrilla-esque appearance.
Think about the personality you want to portray, is she a powerful woman? Meek and unassuming? Calculating? Try and represent that through the way she breathes and moves!
