Answer: you don't. I have never heard of it being done. Sometimes, when you're ready to approach a publisher, you'll write a description for the potential publisher, but this is nothing like a game design document, as it is a marketing tool. If the publisher is sufficiently interested in the game they'll try it out, why would they need a document that describes it when the game exists to play?
When you think you know what you want in your game, make a prototype. If you don't know what to do in the prototype, you haven't thought about it enough. Even after you make the prototype, you'll change it a great deal as you playtest it. So why write a design document?
"The worst form of inequality is to try to make unequal things equal." -- Aristotle
"Always do right--this will gratify some and astonish the rest."Mark Twain
"A designer knows he has achieved perfection not when there is nothing left to add, but when there is nothing left to take away." Antoine de Saint-Exup'ery
"Not everything that can be counted counts, and not everything that counts can be counted." Albert Einstein
"Make everything as simple as possible, but not simpler." Albert Einstein