Design Documents

[ThePortalGuru]

Today, I had this wonderful idea for this moderate-sized game, and I just had to get it down on paper somehow before I lost it. So, I started writing a design document. Problem is, I've never actually written a design document before, so I really have no idea where to start. I've written down the basics of the player so far (Running, Jumping, Dashing, Interacting), but writing it seems like a very daunting task.

Can anyone who's had experience with writing design documents help me out? Where should I start? How can I keep all of the information organized? I need some tips, guys.