Instructions
Insert the Game Shark device into the cartridge slot on your Nintendo 64.
Push your Mario Kart 64 game cartridge onto the top of the Game Shark, then turn your Nintendo 64 on.
Choose "Select Cheat Codes" and press the "A" button.
Browse through the list until you find Mario Kart 64, then press "A."
Select "Add New Cheat Codes" and enter a Game Shark code. You can find Mario Kart 64 Game Shark codes on multiple websites, such as GameGenie, Supercheats and Neoseeker (see Resources).
Press "B" to back out to the main menu and choose "Start Game with Cheat Codes."