Julian,

I think the format of your presentation largely depends upon whom you're giving it to, but I have one thing that I like my employees to do. We like to present scenarios and user response to an app, and it has gone over big with our clients and higher-ups in my organization. Simply presenting these things will show that you have put a lot of thought into the design decisions that you made. You aren't simply flying by the seat of your pants; you are making intelligent, informed decisions.

Since this is part of a job interview, they will inevitably be asking questions at the end. Having already answered the above questions, you will already be prepared to answer their questions.

Finally, be prepared. Don't just show up and give another presentation. You need to make a good first impression.

Cheers,

Brad