As others have pointed out, there's pharmaceutical options to achieve this. I also know nonbinary transfems who use binders, although that's a temporary fix (it works for them bc they're plural / DID and some of the people within their system are femboys or transmasc, so all the personalities have agreed on the compromise that their shared body undergoes HRT, but that they can bind once in a while to present more masc). You could also try lowdose HRT and do workouts focussing on the hips and thighs to feminize your sillhouette. Lastly, there's the mastek option, but that's obviously the most expensive one unless you somehow have an NB friendly healthcare provider and you'd grow boobs first and you'd lose sensation in your nipples post OP.