Ask HN: How to write the financials of your business plan?

We are a team of four founders and are currently working on a project somewhat similar to talkbin. We are in contact with a Business Angel who would like to see our business plan. The thing is, that we currently only launched our prototype and will launch with our first customer (a small restaurant) probably next week. So we have no data whatsoever to make any valid assumptions. I couldn't really find any good articles on the internet about the financials of a business plan. Is there anybody who can share some links to some good posts?