How the USPS flat rate box works?

I am sending a package and I have heard of Flat Rate Boxes, where you pay for the box and then the shipping will be standard for you box when you ship it. I don’t know if i understand exactly what to do after looking over the website. Do I just buy the box and put an address on it and ship it? because i read something about having to scheduale a pick up date? I feel dumb asking, but I don’t want to mess it up.. if anyone can explain to me like what I have to do and what happens i would appreciate it veryyy much:]
thanks:]:D