Use a template from something online. Basically, just cover sales (prospecting), marketing (advertising, media, etc.) and PR (volunteerism, fundraising, networking, etc.). You may also want to address what existing network you have (if any), and how you plan to survive financially (if applicable).

B24 has good tips. I’d like to add that Saydoc.com will tell you if your prospect has read your business proposal in real-time. It shows how much time was spent on each page, what terms they searched, where they were located and more. With that info you can follow up with them when your proposal is fresh in their mind. Thus shortening the process instead of guessing if they read it or not and increasing the chance to close.