So, full disclosure, I had a sorta long-term FWB situation and it ended disastrously… but IMO it was good and healthy while it was going on, at least taking into consideration that we were 16 at the time. So, perhaps you should take my advice with a grain of salt.
I think the most important thing about establishing this kind of relationship – or any kind of relationship, really – is establishing boundaries and communicating about your feelings/needs/wants.
It would suck to get into a FWB scenario and then find out that your partner expected you to be monogamous. Or that they didn’t want to use condoms. Or that they were in love with you. Or whatever. So ALL that stuff needs to be out in the open before you have sex, and you need to talk about it ALL and do some negotiation as necessary.
Aside from regular emotional stuff, also make sure you talk about practicalities, like contraception, STI status, how often you plan on getting together for sex, whether or not you will still hang out as friends, how you are going to tell your other friends (if you are), what the procedure will be if one of you finds someone else you want to date, etc.
Vitally, if your feelings change, you need to talk about it. And you need to make sure your partner knows that (s)he is also free to bring up new feelings as they develop.
I would also recommend that you keep a journal, but I’m big into the whole hippie-dippy lovey-dovey let’s-share-our-feelings stuff, so your mileage may vary. But I’ve always found it helpful to track my moods and unearth my deeply hidden feelings whenever I’m going through some kind of emotional transition.
Wishin’ you the best of luck and some hot awesome sex, cutieface!