Yeah every dentist I've ever been to has said to do it before brushing. The other way around doesn't even make sense, in my opinion.
Doing it before brushing has always made sense to me personally, and is what my dentists have said at well. At least twice a day along side brushing as well. This seems pretty straight forward.
Edited: Thomaslee on 28th Dec, 2019 - 8:36pm
Admittedly, I haven't exactly been the best at remembering to floss. Which I'm suffering for today, with two teeth I need removed but don't have the money for, before then having a root canal for them. Plus fillings for basically all of my molars over the years.