LET ME TEACH YOU THE WAYS OF BOOB MAGIC! My favorite thing about cosplaying is making my boobs huge and here are the techniques that work for me.
I am a B cup that usually gets told that I have the chest of a 12 year old boy online. But don't fear! The use of bras to create fake cleavage is easy and painless.
First off, a Victoria's Bombshell bra and cuttlets are your friends. You can buy knock off bras/inserts anywhere but find one that is comfortable for you. If you want booby body the easiest way is to put on that bra and stick the cutlets on the outer side and under your breast. When you place the cutlets there it'll add volume and cause your natural boobs to be pushed out.
For super duper boobage, I used a Deu bra, a VS Bombshell bra, and cutlets to cover the gaps and create a seamless tit.
For makeup to deepen the cleavage line, use blush between the tits and around the top "half-circle"
Here is a before and after photo of my boobies.
If you have an open shirt and need a little more tittage let tape be your friend. Use the tape to squish most of your boob to the inside of your chest and then tape them upward just outside the open shirt. Then create a sling out of tape and fabric that you can place a cutlet into on the outside of your chest so the look creates a full round boob.