You are the perfect interviewee. You are presented with a pen and a whiteboard and asked to design an events booking system. What do you do?

I was given this task in an interview this morning. A fart would have been a better answer to the one I gave. I scribbled and waffled on about controllers and models and didn't come anywhere near describing a concrete system.

I could talk through the steps required to create an app in Rails but have no idea where to start as far as designing one on a whiteboard.

What would the perfect interviewee do? Would they talk about the user? Would they sketch out a UML diagram? Should the design mention classes at all or should the interviewee talk about the app at a higher level of abstraction?

What would the perfect interviewee do in five minutes to satisfy their interviewers? (That is, what do I need to learn to be able to communicate my ideas clearly via a whiteboard?)