I am not sure how I would bring it up to someone.
I had a great FWB relationship with someone. He was a friend of a friend, we hung out just me and him and we got it on one night (we were NOT drunk). Then we would make each other booty calls over a certain period of time.
One day I called him for sex and he told me that he was interested in someone and we could no longer hook up. I was a little upset...I had no one to hook with anymore, no more sex, lol.
We were great friends but we were totally different and I could never have dated him. We had lots in common but I never could have gone out with him. He did drugs and he wasn't living the way I wanted to live and planned to live my life.
It started spontaneously, went smoothly, and ended wonderfully. That is my personal experience with FWB. I never had any other FWB after him. Hope my experience could bring you something. I am ot quite sure how I would go about it if I had the intention of starting that type of relationship. It just sort of "hapenned" for me.