Look at other people's projects if they post source. Then, plan whatever game you want to make. Plan every intricate detail of it. Then start building tech demos to figure out how to make certain elements of the game work the way you want them to. Then make a demo first level as a prototype. From there you can add more to the interface, but mostly you'll be constructing levels and creating objects for them. Then after the game is mostly complete visually and input-wise you should start looking into sound. Recording sound effects and/or buying collections of royalty-free sounds. Then with music the same thing goes, or you could get somebody to write it for you.
Crazy Donut Productions, Current Project: KillZone
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