If you are an FTM, a cosplayer, or you just want to flatten out, Then I can tell you how to make a chest binder. (In the top picture I am trying way 3, because I am a 34C. In the bottom i am wearing a regular bra. )
Way 1:(For smaller busts) Take an old pair of leggings, pantyhose, or spanx, and cut out the crotch. You can also cut the sleeves. Pull your head through the hole and put your arms through the leg holes.
Way 2:(For medium bust size) wear the leggings you made, and take out 2 tight camisoles. put one on backwards and roll it up in sections so that it looks like a bra. Do the same with the other one but wear it forwards.
Way 3:(For larger busts or more coverage) Wear the leggings and the camisoles but with a sports bra underneath and the leggings on top.
Cautions: Like a regular binder, do not wear for more than 8 ours, and never sleep in it. If you can’t breathe at all take it off. It should only impair breathing by a fraction.
Thanks for reading, and I hope this works for you!