A proposed application has twenty modules. Each sounds useful, but the team cannot explain what the first user will complete on launch day. Start with one end-to-end journey.
Define a finish condition
An internal request tool might let a staff member submit a complete request, a reviewer decide it and an operator record the result. That is a usable slice. Building half of each module may leave nobody able to finish the work.
Identify the roles, information and exceptions needed for that journey. Keep later ideas in a visible backlog so they are not forgotten or quietly treated as part of the first commitment.
Include the operating essentials
A first release still needs appropriate access, useful errors, recovery and a way to support users. These are part of completing the journey, not optional polish that can always wait.
Ask users to test a realistic example in the prototype. Watch where they need information or permission that the design does not provide.
A clear first release gives the business something concrete to accept and a better basis for later decisions. Bring the task that would make the initial system useful, along with the person responsible for it. That is a stronger starting point than trying to estimate every possible feature at once.