The times that I've had a FWB, it's gone like this:
1. We meet through a common social circle
2. We hookup, usually at/after a party or a night out
3. Afterwards, we have the inevitable "talk" about what it meant
4. We both agree that it was fun, but we're not looking for much more
5. Rinse and repeat
It's tough to put into words that kind of proposal before the deed is done at least once, in my opinion.