You need to be emotionally literate and self aware. You need to acknowledge that there’s a possibility you will get more emotionally involved than you planned to, and know that you’ll be able to own this (by telling your partner, working your way over it, or ending the situation.) You need to be able to communicate your needs and desires and be upfront about why you’re doing what you’re doing. You need to be looking for a FwB situation because you actively want this type of situation - and not just because you feel too vulnerable to trust somebody in a committed relationship yet: this is important, the more casual and fluid nature of an FwB arrangement will only make you feel more vulnerable and insecure if that’s the way you already feel.
I have several. It works well for me and meets my needs as they are at the moment. I didn’t plan for things to be this way - I came out of an LTR and started dating about a year later. I met somebody who I had a few dates with, after which he said he really liked me but knew he was still a bit hung up on his ex, didn’t want to lead me on and didn’t know when he’d be ready for something more serious. At that point I said I was happy to just date casually and see what happened. Two years later, what’s “happened” is that we are genuinely good friends who have sex. We still go on dates, we go out, we go away for weekends together, we spend time with each other’s friends. But we lead independent lives and there’s no expectation on either side that this will change. We check in with each other regularly about how we feel: “are you still okay with whatever it is we’re doing?”. “Do you feel you need anything else from me?” We both have other people we have similar arrangements with - for me, once I realised that the independence and fluidity works for me, I felt a lot more free to explore with other people.
I said on another thread that in a good FwB situation, there’s nothing wrong per se with having feelings for each other as long as you’re both in the same page about it. I’m tactile and affectionate with all my long-term FwBs and they with me - it’s really the whole point of the rodeo: we genuinely like, care for and are invested in each other. We probably love each other a bit, in that way you love a close friend. Just because somebody doesn’t want a fully committed monogamous relationship with the expectation that this will escalate into living together and marriage etc, doesn’t mean they don’t want care, affection, tenderness, intimacy etc. But you need to be able to recognise the limitations of the arrangement and enjoy things within those confines rather than expect or seek something more.