You researched a car rental agency and created a data model for the rental of cars.

Utilize the work you previously did, as well as preliminary interviews, to create a prototype for a system that rents cars.

Why do you need to do more interviews? This is so that you can develop an interface as well as functionality that the user wants and needs. Remember, aside from process functionality, the user will determine the success or failure of a system. What do they want the interface to be and act like? What format do they want to handle data in?