Start with the workflow.

Before choosing a platform, describe what someone is trying to accomplish. What starts the work? What information do they need? Where does the process become slow, confusing or repetitive? A workflow makes those questions concrete.

Choose a single journey that can stand on its own. A focused first release makes it easier to test assumptions and learn what the product needs next.

Write down acceptance criteria in plain language. A successful task, a recoverable error and an understandable handoff are better starting points than a long feature list.