A proposal letter isn't just a summary of your full proposal. It's a persuasive introductory document meant to intrigue a reader enough to find out more.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. We'll outline basic formatting techniques that can be used thoughtfully and strategically to strengthen your proposal. Business contract lawyers typically complete the work in nine simple steps. Introduction to Proposals and Proposal Documents. Murray, J. W. (2008). Each proposal must be planned out before a word is written. A business proposal letter is a document sent to a prospective client or partner, introducing your services or requesting an opportunity.