Include Signed Contracts and Offers
There is nothing quite as strong as a signed contract to substantiate your assumption or claim that a customer will buy your product or service. A written offer is almost as strong in terms of credibility, certainly as strong as a contract.
No example is provided here because the nature and type of contracts varies so widely. The important thing is that you research your particular Industry and get to know what the Industry standard is for contracts for your business.
If you are bidding or responding to calls for proposals for government or Industry contracts, the buyer will usually dictate and provide the contract. Most government agencies work from boilerplate contract templates that they adapt to your specific situation. In these instances, your only real control lies with how you write your proposal and how thoroughly you read the contract before signing it.
If you are not experienced at writing and reading contracts, I strongly recommend that you engage a competent lawyer to assist you. In spite of all those bad lawyer jokes, a good lawyer will save you money.