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Just as the entire project uses a design process, you should develop the graphics and sounds using a process. Breaking down the development of graphics into discrete stages helps if you are working on a team, but even more generally it will help you keep organized and focused. The following stages are generalized, and will, we hope, get you going until you develop your own workflow. If the entire idea of developing graphics for your game makes you a little uneasy, don't worry: there are a lot of great sites (e.g., OpenGameArt.org) where you can download sprites, tiles, and other graphics for use in your game.
In this stage, you will want to draw a lot of versions of everything that will be in the game. If you are like me, I have to stop myself from just jumping into a graphics editor and starting to mock up what I want the application to look like. However, if you are working on a project of any real scale, you really do need to sit down and think through all the necessary assets for your game, and what is needed to implement the overall vision.