Instructions
Insert the GameShark cartridge into the Gameboy. Insert the Pokemon Gold cartridge into the GameShark if you have a GameShark with a slot for a Gameboy game.
Turn the GameShark on. Turn the Gameboy on.
Select "Pokemon Gold" from the list of available games. Select "Add New Game" and type "Pokemon Gold" into the GameShark with the onscreen keyboard if it is not on the list.
Select "Add New Cheat" from the menu.
Use the onscreen keyboard to enter this code: O183D8CF
Press "Start" on the GameBoy to start the game if you have a GameShark with a game slot. Follow the onscreen instructions to switch to the game if you have a GameShark without a slot. Remove the GameShark and insert Pokemon Gold when prompted.
You will instantly battle a Mewtwo when you start the game. Fight and catch him in the normal manner, using a Pokeball when he has lost nearly all of his hit points. Use a Master Ball for the best results.