Don't.

I'm in the exact same situation as you are except I'm actually fucking my ex and have been for a few weeks. It sucks, he's very attached and acts like we're still dating, except he's less considerate.

I've had fuck buddies with just a friend before except we weren't really friends, kinda more of a booty call. That was alright except his junk was mad small, we didn't actually fuck either..just fooled around. It wasn't too bad until he wouldn't stop calling and shit. He was the head of the wrestling team or what ever, and not my type. Lol.

I don't suggest banging someone you've already had a relationship with.