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).
(Also known as the Concept Document). This is really the bare minimum you should have for your project, whether its an indie game, a small modification, or a total conversion.
The Game Concept Document is your mini design document - its the pitch, and overview of your project. This is vital in the short term to fit what you need for planning your project. Depending on the size and complexity of your project, a GDC may be all you need.
For the full size document, then go to the link below. This only advised for full scale mods and indies, due to the shear size of the documentation you're being advised to write out.
You can find the Design Document download here: