These Files:
http://www.filefactory.com/file/ecab25/n/Wii_Fix_Tools_zip
IOS16-64-v257-prod.wad - Google It and place it in the 'WAD' folder
Step 1:
Update your Wii to the latest version.
Step 2:
Using "The Homebrew Channel", open "WAD Manager".
Step 3:
Press "A" on the Wii Remote once.
The Wii should detect a SD Card.
Press "A" again to see a list of WADs on the SD Card.
Use the "D-Pad" to select "IOS16-64-v257-prod.wad".
Press "+" to install the WAD.
Once installed, press the "Home" button, to restart you Wii.
Step 4:
Using "The Homebrew Channel", open “Custom Manager".
Using the same method as in "Step 3", install the "cIOS_Fix.wad".
Once installed, press the "Home" button, to restart you Wii.
Note:
If that fails to install, just use “WAD Manager” to install it like in the youtube video.
Step 5:
Using the ‘Homebrew Channel’, open ‘IOS Downgrader’. Just follow the instructions on the screen and wait until it finishes installing the necessary IOSs.
STEP 6:
Using "The Homebrew Channel", open "Install Shop IOS51".
Use the "D-Pad" to select "Yes", and install "IOS51", by pressing "A".
Use the "D-Pad" to select "Yes", and install the IOS Patch, by pressing "A".
Use the "D-Pad" to select "No", and DO NOT install the "Wii Shop Channel".
Step 7:
Using the ‘Homebrew Channel’, open ‘WAD Manager ’. Install ‘WAD MANAGER 1.3’.
Step 8:
The Wii is now able to install WADs and Homebrew Apps should work properly now.
Optional Step:
If you use “WiiGator” or the “Backup Loader”, using the Homebrew Channel, install ‘cIOS Installer’. This updates the ‘cIOS_Fix’ that was previously installed.
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Taken from RadioShadows's guide.