Mods are typically released as patches to the original game, for which you need the original disc. In order to run them, you will need an homebrew app named Riivolution, whose job is to patch the game files on the fly. If you don't own the game, you can use a backup loader or Dolphin, and for those you can find already-patched ISOs on the internet, but this will not be covered here for legal reasons.
Installing Riivolution[edit | edit source]
Riivolution requires your Wii to be able to run homebrew applications. A good guide for enabling homebrew can be found here. Note : the app only works with original game discs, burned ones won't work.
Download Riivolution from here and extract it on the SD Card.
Now place the mod of your choice on the SD Card as well. They usually come with everything you need to load them (including Riivolution itself, but you already have it). Once done, put the SD Card in your Wii, open the Homebrew Channel, and from there start Riivolution.
The program will then load. Feel free to press Install to add a Riivolution channel to the Wii Menu. Enable the mod of your choice, then select Load, and the game will start.