So, I have two FWB. One is my ex from a 5 year relationship. I bascially approached him, and asked him if he would be interested in a no strings relationship, in which both of us were free to see other people..... I wouldn't say it's been entirely plain sailing - we have both had our jealous moments - but I think we have established an equilibrium now whereby we don't pry into what the other is doing, during the time we are not together. The benefits are good, because we both know each other well and there is strong sexual comparability. But obviously one has to get to a position of strong emotional resilience, for this to even be an option.
My other one is someone I met for a date once when my ex and I were on a break - we stayed loosely in touch via social media, and when we were both single we resuned texting, and one thing led to another.... While I would be open to a full relationship with him, he has always made it clear that isn't on the cards - but he is a good friend - the benefits are secondary in our friendship I would say? (Though the sex is also good).
So my advice would probably be, to review your former connections and decide if there is anyone from your past who you know you ar sexually compatible with, and bascially be up front about it? I am stereotyping wildly I know, but from my experience few single guys will turn no strings sex down.....
My best friend does it another way, and meets her FWBs through OLD - she basically kept her same profile (which says she is looking for a relationship) but when guys message her and say they were looking for NSA fun, at that point she says she is up for it (to the ones she finds attractive). She's pretty bold though - she will invite guys over to her house for an agreed "scene" - she sends me their details beforehand (and makes sure they know she has done this) and sends me a proof of life text afterwards, so there are certain safety checks in place! But there are risks involved too, obviously, and I wouldn't exactly recommend this approach.....
She has actually met her current partner in this way - they started as FsWB, and fell in love over time. I have to say he isn't my favourite person in the world - I find him quite narcissistic and untrustworthy to be honest - but that's nothing to do with the fact they met as FWBs, it's just the man himself....