She’s one of your best friends; you can play it by ear. You don’t need a plan.
You might, however, start with this: “Sarah, you’re one of my best friends. In my mind, there is nothing better than to fall in love with and marry your best friend. Those are the lucky couples. One of the worst things, though, can be falling in love with your best friend, but your best friend doesn’t fall in love with you back. Can we agree right now that if we decide we want to go back to being friends without romance, we’ll be honest about it, say so, and accept it? Because of course there is the third possibility, which is that neither of us feel romance for the other but neither wants to hurt the other by saying so and so no one says anything. That would be silly and pathetic at the same time.”
“No one is more important to me than you are, but this might not work out. It certainly may not work out in anything like a hurry. We’re going to have to wait and see. I promise you, though, that if you decide a romance is not in the cards, I will be a good sport about it. I promise you that I will respect your time line, and if you want a commitment that I’m not ready for, I will not ask you to wait around for me. I hope we do hit it off, because whoever marries you will be one lucky man, but I want to look out for you and the friendship we already have, so I’m saying that up front. If you want to make me happy, find the man who will make you happy, even if that isn’t me.”
That doesn’t guarantee that there will be no awkward stretches ahead of you–the rules of romance will not be re-written for you!–but even so she’ll thank you for that, I think.
