In legal terms, a contract is any agreement between parties to exchange things of value, such as goods and services for cash. A simple handshake agreement is legally binding, but its terms are not always provable if a dispute arises. However, you don't necessarily need pages upon pages of legalese. A basic contract agreement often works best for a simple business transaction.
Describe the other party's obligations in detail. In the case of out brochure writing example, this might explain that the client is responsible for providing the copywriter with business information in a word processing document to describe the business and the information to be included in the brochure text.