Step 1: Choose a descriptive title
Others will see this title appear in the list of active projects, so make sure it describes your project well. Once you click "Start new project page", you'll be presented with an edit window and some more instructions.
Step 2: Fill out the form
Make sure that you capture the basic idea, but don't worry about writing it all up. You can always come back to add information later. This is a wiki after all.