Things You'll Need
Instructions
Find a large SD Card. Although the Wii does not support internal hard drives, it does support flash storage via SD card. The current firmware (as of 2009) supports up to 8GB of flash storage, so any SD card that reaches (but does not exceed) this capacity is acceptable.
Open your Wii's data input port (at the front of the console). You will see a long skinny port beneath the reset button. This is where the SD card goes.
Place the SD card in the port, label side out, and allow it to "click" into place. Then close the Wii's data port door.
Power on the console and click on the "SD" icon at the bottom of the screen. Nintendo will need to format the SD card, which will take several minutes. Once formatted, all previous content on the card will be lost, so it is important to back up any data you had on the card previously before using it in your Wii.