How to measure my chest for a binder?

I found out that my school has a program that can get me a binder for free through a program that they do without my parents knowing (yay). I have done A LOT of research on chest binders, even before I knew I wanted one. I know how to safely use them and I have friend's I can ask as well. I just don't I know how to measure myself for one. I can't find any good guides, and I'm not sure if I can trust the guides anyway, as I'd rather get information from people who have had experience.