Describe journeys and acceptance criteria
Start with users, roles and the actions they need to complete. Write user stories supported by concrete examples and expected outcomes. Include failures, permissions and alternative paths as well as the ideal flow. Acceptance criteria should make it possible to decide whether the delivered feature works as agreed.
Document constraints and dependencies
List platform support, integrations, existing data, security needs and operational requirements. Identify decisions still to be made and who can resolve them. Provide available API documentation and sample data where appropriate. Separate assumptions from confirmed requirements so that uncertainty remains visible in the estimate.
Use the document throughout delivery
Keep the specification connected to a prioritised backlog and update it when decisions change. Define the first release separately from later improvements. Use demonstrations and acceptance reviews to check the result against the agreed behaviour. A specification should support collaboration and controlled change rather than become a document that nobody maintains.