How do you approach a girl about becoming friends with benefits?

Be direct about it and make it as not awkward as possible. I've used more or less this exact line to set up every FWB situation I've ever had:

"okay, real talk: would you be down to be friends with benefits? We're both fun, attractive people, we clearly have a good time together, and we can trust each other, so what do you say?"

The point is to be direct- let her know you're interested, but that you don't need anything from her. If she says no you have to be okay with being just friends. FWB is a tricky scenario and a girl is going to wonder how it will affect your friendship- you have to get her to trust that nothing is going to change, except you're going to have a lot of fun, spontaneous sex.

It's always a good idea to lay down some ground rules too. I didn't really do that with my first FWB years back and we ended up dating after a few months which was NOT what I had intended. Figure out what you both want and set some hard rules that fit those guidelines.

Best of luck man, FWB can be really fun =D

I think it's all about finding the right type of girl who is willing to have this type of arrangement. When it's obvious that both of you are attracted to each other, just tell her that you are not looking for anything serious. Once you tell her, you'll have a better idea of whether she wants to become friends with benefits. For example if both of you keep hooking up, but don't seem to be in a relationship, then you can assume that she's fine with it. If she bails when you tell her you don't want anything serious, then she's not the right type of girl for this sort of thing.