Hey, when in doubt, it is always safer to go with the larger sized binder. If you get a size too small there is a greater chance of getting injured while wearing it. Just look at the size chart for whatever binder you're getting and see what size seems like the best fit.
As for the way to put it on, I don't really understand what they're saying either. The first time I put on a binder I tried to get it past my shoulders and kind of put it on in a way a t-shirt would go on. Needless to say, I had a lot of trouble and I might have flopped around like a fish a little bit. You can try my method first (0/10 would not recommend), or you can attempt to put it on up from the legs. You can do the latter one in the beginning, but the binder will stretch out, and you'll be able to put it on over your head easily after. It gets easier to put on, trust me--I really would have needed those words the day I got my first binder lol.
Good luck.