Start with the work that needs improving
Before listing features, describe the problem. Where does the team lose time? What information is missing? What makes customer service harder? A concrete description of the situation is more useful than a list of technologies. Include the people doing the work every day, not only those managing it.
Define users and responsibilities
A platform may be used by customers, operational staff and administration. Each role has different tasks. Describe what each role needs to see and do, and who is responsible for keeping information up to date.
Separate essentials from later phases
The first version should solve a specific need with features that fit the available time and budget. Separate launch essentials from features that can follow later. For each feature, describe how you will know it works correctly. This also makes time and cost estimates clearer.
Take stock of systems and data
List the tools you use: accounting software, spreadsheets, websites, databases and external services. Each integration needs documentation, access and a contact person. Do not assume a connection is feasible before it has been technically assessed.
Plan what happens after launch
Define who will use and maintain the platform, how issues will be reported and how changes will be prioritised. Delivery, documentation, training and support should be discussed during planning. A good project brief should cover the problem, users, processes, integrations and goals for the first version.