Have your tool(s) in mind--your prototype should mimic the final design of your project without you having to learn the tool just yet.
Think about how users will move through your story. If there is a "click," then start a new page.
Make your work legible. Use color to create consistency or emphasize certain parts of your design.
Draw on paper and digitize, or draw with some kind of digital pen/software.
We will be workshopping these, so take advantage of the time to test your ideas. Also, be prepared to tell us directly what tools you are thinking of here.
Do not sketch something out five minutes before class.