Well step number 1 is that you never actually say "friends with benefits" well at least not before you have already hooked up with her multiple times, and even then it would only be if she brought it up.
Learn that this is the real world and not everybody wants a relationship and even if they do, relationships just sort of build until one day you decide to be exclusive. By your response its like you want her to sign a contract on exactly how your relationship between the two of you will play out.
Never, and I mean never bring up any talks about what your current status or situation with a girl is. If you bring it up early she is going to label you as a friend, and she would be right since you haven't been sexual with her, and from that point on your labeled friend and friends don't hook up. If you bring up short or long term relationships your going to scare them off especially if you haven't hooked up with them.
This isn't high school, not everything has to have a status or a label, go out with her, make a move, escalate, and close the deal. Then just repeat over the next few weeks, and there you go friends with benefits. Don't lie to her, just don't bring up what your situation is or what you think of her. If she brings it up, try to avoid labeling the situation and if she pressures then just be honest with her then, but by that point it doesn't matter much.