Instructions
Place the game cartridge into the Nintendo 64 that you need to erase game data for; but do not power on your Nintendo 64.
Place the memory card you are using into the controller. Push the game down into the memory card slot on the bottom of the controller. Make sure the controller you are placing the memory card into is the player 1 controller.
Press the "Start" button in the center of the controller and hold the "Start" button down. Turn the Nintendo 64 game console on and once it is on a memory-card screen will appear and you can now let go of the "Start" button.
Delete any saved game data as needed.
Restart the Nintendo 64 and select "Options" on the main menu of the game data you wish to delete if a memory-card screen doesn't show up in earlier steps. Some games save data on the game itself instead of the memory card. Choose "Delete Game Data" under the "Options" screen to delete unwanted data.