Start with one useful task
Write down who the product is for and the task they need to complete. For a booking app, that might be finding a service, choosing a time, and receiving confirmation. Use that journey to decide what belongs in the first release.
Separate essential work from later ideas
Sort features into essential for the core journey, needed to operate the service, and possible later improvements. Administration, payment handling, privacy, and support can be essential even when they are less visible than the interface.
Define what ready means
Agree how you will review the work: which journeys must function, what devices matter, who supplies the content, and who handles launch and support. A shared definition of done makes a proposal easier to assess.
Bring the unknowns into the brief
You do not need to choose every technology before speaking to a development partner. Bring the business goal, any existing tools, your constraints, and the questions you cannot yet answer. That is a useful starting point for discovery.