Start with the workflow
Describe the work that happens today: who does it, what information they need, and where the handoffs break down. A concrete example is more useful than a long feature wish list.
Name the first useful release
Separate what is essential to complete that workflow from what can follow later. Define what a user must be able to do from beginning to end.
Agree on acceptance
Write down how you will check the result. Include roles, real data shapes, integrations and failure cases. A shared definition of done makes estimates and delivery easier to discuss.