Know how the business operates to be able to test a change in an area. Is this information to be gained via documentation or key users?
The business processes may change, and as the UAT is implemented in a separate environment, you will need to know the new business process changes, including inputs, outputs, and outcomes.
You may want to document this understanding and associated data capture using a classic structure. However, the downside of this is it does not lend itself to multiple scenarios. In this case, you will need to support any process definition with the data variations that are required, both for input and output.
If the project is largely revisions and additions to an already existing system, you may want to capture the existing process from production if the knowledge is not already available. You can use this to create text style instruction cases or even how-to videos, which users can refer to alongside data scenarios.
The UAT management tool should be used here to provide a) a repository for these assets and b) the ability to organize and hence reuse them.