Measuring cup size

So I went to Victoria Secret and they measured me and were way off the bra was many sizes too small, and it was confusing since they just wrap a tape measure around your bust and I don’t I don’t really understand what they did. How would I measure myself accurately, I’m in need of new bras, they have gotten way bigger since I’ve gotten pregnant and with COVID we aren’t able to try them on and I don’t wanna get a size too small. I know I need the tape measure but where would I start from? Under the breasts? Or?