Start with the work
A useful project brief describes the people using a system, the tasks they need to complete and the constraints they face. Capture a few examples of the work as it happens today. These examples give the team something concrete to discuss and test.
Agree what completion means
Turn important requirements into observable acceptance criteria. Describe the expected inputs, outputs and exception cases. Decide which deliverables are included: source code, deployment, documentation, training or a support period. Record changes as the project develops.
Keep progress visible
Regular demonstrations help connect the build to its purpose. Review a complete workflow, collect feedback and decide what changes next. A small, working increment often provides clearer evidence than a long list of features marked as nearly complete.