Familiarize yourself with game you'll be working with. Remember that the action in a game can be extremely dynamic -- one second, the protagonist might be traveling through a peaceful countryside, and the next second he could be battling a dragon. Your music should be equally dynamic. Conceptualize what sort of music will fit the theme of the game, and have musical ideas ready for any situation.
Purchase some notation software, such as Finale or Sibelius. This will allow you to electronically jot down your ideas as you have them, and play them back in real time. These programs will let you create full multiphonic scores, and save individual pieces as separate files. You can print the sheet music or save songs as MIDI files that can be used with sequencing software.
Consider the types of sound effects the game will require. If the game is a futuristic shooter, you'll need to create sounds like gunshots, laser sounds, and general splat and thump effects. If you're scoring a racing game, you'll need to record engine sounds, crashing sounds, and crowd cheering.
Purchase sequencing software, such as ProTools, Cubase or Digital Performer. In addition to their sequencing functions, these programs will allow you to record live music. The only limitation at this point is your imagination. You can record anything you wish. Alternatively, you can import the MIDI files you made with your notation software and assign virtual instruments to play the parts you have written.
Test your sonic creations by playing them with the game. Play through a game level while listening to the music you have created. Ask yourself if the music is appropriate for the level, and if it serves to enhance the immersive experience. Make any necessary adjustments or changes. Chances are good that at least a few aspects of your original score will require some revision.