In this article, we discuss how to write a business proposal letter and we offer sample letters and templates you can use as a guide. This website has a great example of a business letter format.Finally, remember your goal. Fill out and return to the Harris County Purchasing Department ONE (1) complete proposal form in an appropriately sized envelope or box. Discover the secrets to creating a winning catering proposal in just 6 easy steps! Here are expert tips and a free template. Here's your crash course. On writing and designing a business proposal. Keep the letter short and descriptive, up to one or two pages. Use the overview to provide a brief summary of the reason for the proposal, and how you can best support the client's needs.