[kaiserama]

If you haven't done this already I would take as much time and care mocking up drawings and explaining how your system will work in lay terms. This will be good for 2 reasons, 1) you'll continually refine your concept and 2) it will provide a basis for explaining the system to someone else.

In my experience dealing with clients who come in with an idea, often times that idea isn't fully formulated and we end up making huge architectural changes downstream which are extremely costly. I consider myself at least somewhat business savvy so I can spot potential problems and generally bring them up and get them corrected ahead of time, but if you're dealing with someone who is going to build off spec you probably won't have this safety net.

Like RiderOfGiraffes says, if you don't provide a developer enough vision of the product you'll likely end up with the wrong modules. For instance let's say you're setting out to build a formula 1 car and you send a spec out for a steering wheel without mention of it's purpose. You're probably not going to get what you wanted.

[ericseidelman]

agreed.

I currently have detailed wireframes, a UI design mockup, and a complete overview description. So those should help.