The first thing I do once I have got an idea for a game is define the game states that will be used. I then code stubs for each function of the game states (rendering, mouse move, mouse up and input)

I then do a detailed introduction screen which in effect becomes my design document as I try and describe what the game is all about.

I then use existing images to create the game itself. (I mostly use Reiner Prokiens images)

Lastly I try get everything working smoothly and effectivly - also try do some optimisations.

I use a notebook to write down my ideas as they come up and make notes on ideas during the full process. I never do a design document as I then feel to restricted in my options.