Basically once you bang them, you either invite yourself over to their place or invite them over to your place and pretty much rail them every time...no dates, no girlfriend/boyfriend activites, etc...nothing needs to be said, she knows what way you are trying to take the relationship.
She will let you know if she is down with that or not...usually you can get away with it 3 or 4 times if she isn't with it and then she will be give you the whole "I really like you" speech and basically tell you she wants you as more than just a sex partner...
Then 1 of 3 things can happen:
1) You can try to ignore what she said and just keep doing it, which in my experience doesn't work that often if she wants a relationship with you...she will just start flaking on you unless you offer to take her out somewhere
2) You can get in a relationship with her and carry on as before but with the additional relationship activities added to the mix
3) You can drop her and tell her you like things the way they are and if that's not something she's OK with then things will have to end.
Unfortunately for me, every time I've ended up just wanting a FWB they always told me they wanted more within the first 4 or 5 "dates", and I had to drop them because I could tell they were going to start getting clingy which I didn't want...