Instructions
Go to CodeJunkies.com on your computer to look up Action Replay codes for the Gameboy game of your choice. You'll find a list of available codes in the Game Boy Action Replay codes section of the website.
Copy down the codes you want to enter into your Action Replay cartridge.
Put the Action Replay cartridge into your Game Boy.
Enter the game's name when prompted with the on-screen keyboard. To access the keyboard, use your directional pad to highlight the keyboard and press the "A" button to begin typing.
Use the directional keypad to select letters on the keyboard and enter the cheat codes into your Action Replay cartridge. If you need uppercase letters or special characters, press the left shoulder button to toggle between lowercase, uppercase and special characters.
Enter the game's (m) code before entering any of the individual codes. This code is unique to each game and will appear at the very top of the code list on Code Junkies. Use extra care when entering the (m) code, because if it is entered incorrectly, none of your very long codes for the game will work. Press the "Enter" key when finished to save the code.
Enter a cheat code. First, you'll need to give your cheat a unique name so that you'll be able to recognize it. Next, you'll need to enter the exact code of hex characters for the code -- just as you did for the (m) code. Press the "Enter" key to save your code after you've entered all of the characters and double-checked it for accuracy.
Enter additional codes by repeating the steps above. You can enter as many codes as you want for each game.