Instructions
Insert the Action Replay disc into your GameCube with the power turned off. Close the disc tray. Plug a GameCube controller into Port 1 of your console.
Turn on your GameCube and immediately press "B" on your controller. Hold the button until the menu screen for your patched firmware loads (it will look completely different from the regular GameCube menu screen). If the regular GameCube menu screen loads, turn the console off and on again, pressing "B" just as the console is first starting up.
Select the game you wish to play from the Action Replay menu then choose the game-enhancing cheats you want to use for that game. Select "OK" and press "Eject" when the message "Swap Discs" flashes onscreen.
Remove the Action Replay disc and insert an original or backed-up copy of the game you selected. Close the disc tray until it clicks to boot your game with Action Replay cheats activated.