Produce the design documentation.
For the conceptual design (architectural or high-level), identify the interface metaphor to use, the interaction type(s) to employ, and the interface type(s) to follow. For each of these, be sure to explain why you choose what you did.
For the physical design (detailed), produce a prototype. The prototype should closely represent the actual user interface, so use a software tool (such as Word, PowerPoint, Excel, etc) to create it. Explain how your prototype follows your conceptual design goals.