any tips?

Some of my friends use two sports bras

going off of the other comment, one sports bra on regular and one on backwards makes you pretty flat. Whatever you do though, DONT USE ACE BANDAGES. They will tighten a lot and are known to crack ribs and make it hard to breathe. I would suggest doing some research on the subject to make sure you are binding safely.

yep. i have a friend who at some point spent the night in the hospital because they worked out while binding with bandages. I can’t imagine it was fun

I have a way. You wear a sports bra, put on a vest and tuck it into the bra, put a loose shirt over it and u have a temporary binder

How to MAKE one?

Well, You could knit one with some Lion Brand Wool-Ease Yarn, I tried knitting once, Worked like a charm xD