Well, lets face it. Writing a design document is difficult, and very time consuming. On the projects that I've worked on, I've used this PDF made by the kind guys at CONJURE, (which I believe is the best one available, at least freely, to the community).
THIS IS ADVISED ONLY TO BE USED FOR INDIE GAMES AND FULL CONVERSION MODIFICATIONS DUE TO THE SHEAR SIZE AND DETAIL OF THIS DOCUMENT.
The Game Design Document is your main design document - this is the time consuming beast that will explain exactly what you're putting into your project, and how the game will work. This includes asset lists, time management.. its all in here, and worth reading through.
Bear in mind - some sections may not apply, or some areas may repeat themselves. I
I'd advise doing the Game Concept Document first - it may in fact be all you need for your project, but for full projects you really need to create something like what has been set out in this template document.
You can find that at this link: