The first version of an internal tool should let a defined role complete a real task. A collection of partly connected screens is harder to evaluate because nobody can finish the work in it.
Choose the loop
Specify the event that starts the work and the evidence that closes it. An example could be a submitted request that reaches a recorded decision. Name the person responsible for operating this loop during the trial.
List the records and state changes needed between those boundaries. Include the most relevant exception, such as missing information or a rejected request.
Make exclusions explicit
Write down which adjacent tasks remain in current systems. A first release may use an export rather than a complete reporting suite. It may support one team before supporting every department.
An exclusion is acceptable when the team knows how to handle it. An undocumented gap becomes manual work discovered during use.
Define the release check
Run a normal item and the selected exception from start to finish. Verify permissions, recoverability and the output the next role needs. Agree which observations will determine whether to expand or revise the tool.
For managing the scope after that decision, see scoping an MVP internal tool.