Ah hello I can help with this one! You need to measure around their chest, it's important to make sure you are not going too tight when you do this, it is much much better to have a too loose binder than a too tight one. Also when your friend does get the binder, please have them check in with you at first to make sure it is not hurting them and is not too tight/restricting. Too tight binders genuinely can be dangerous.
(I'm keeping this pg, but I really hope you know what I mean when I say chest.)
(Ok that's it. NB squad out!)