Then, deliver your polished proposal. Save it in a PDF file or print it out, or both. The best delivery method for you depends on your relationship with the party who will receive your proposal. It's common to attach PDF files to email these days, but for your situation, it might be most impressive to hand_deliver a nicely printed and signed proposal.
It's all about the relationship first. Many newbie business owners give a proposal to anyone that they think seems interested in their services. That is an easy way to waste your time. Before you submit a proposal, you should have already developed a strong relationship with the buyer and showed the value that you are going to deliver. Your proposal should be a reiteration of that value and relationship and should only be submitted when you know that you are going to make the sale.