Unless you're really big chested as a gal, I would suggest buying a binder. Please don't use the Ace bandage or similar method as that can cause a lot of damage to the muscle and tissue underneath. My chest is thankfully on the smaller end of the spectrum (B cup) so using a binder really helps flatten my chest considerably! You'll never be able to flatten your chest completely but technically guy's don't have 100% flat chests anyway. When I wear my binder, small bumps still show but since the binder smooshes them out, they appear as flat bumps instead of round bumps, if that makes any sense? Haha so they look like pecs once the rest of the cosplay is on. I buy mine from here
but you can buy them anywhere. Just search "ftm chest binder". I suggest buying a half-chest/crop top one since the full length ones seem to ride up throughout the day and look all weird and bunched up around your stomach area. Hope this helps!