1. Download and install ModMii for Windows
http://gbatemp.net/threads/modmii-for-windows-official-support-topic.207126/
2. Run it and go to ModMii classic mode (if you accidentally run ModMii GUI instead, ModMii classic is the last item on the main menu)
3. Type "I Agree" (without quotes) and press Enter.
4. Go to Download Page 4 (press 4 and then Enter)
5. Select the three different cMIOS options (Without quotes type "10", press Enter, type "0.2", press Enter, type "5", press Enter.)
6. Press Enter again to take you to Download Page 1.
7. Select BC for download (Type "BC" without quotes and press Enter)
(steps 8 and 9 are optional if you don't already have a WAD manager)
8. Hit enter again to take you to Download Page 2.
9. Select "Yet Another Wad Manager Mod" for downloading (type "YAWM" without quotes and press Enter)
(if you so prefer you can also download WiiMod (WM) or Multi-Mod Manager (MMM) which have WAD managers built in as one of their many features)
10. Download files (Press D, Enter, Y, Enter)
11. Wait for ModMii to download and build the files
12. Press the letter O and then Enter to open the folder called "COPY_TO_SD" where it has downloaded all the files.
13. Make sure you have an SD card connected to your computer and that the "lock" switch is off.
14. Copy all files to the SD card (CTRL+A to select all -> right click -> send to -> select your SD card drive)
15. Take the SD card and put it in the Wii.
16. Turn it on and go to the Homebrew Channel and open your WAD manager.
17. If it asks you, tell it to use IOS249, NO emulated NAND and the SD card.
18. Select the BC-NUS-v6.wad file and install it.
19. Select and install one of the other cMIOS WAD files (they have RVL-cMIOS- or RVL-cmios- in the name).
20. Use the home button (the house) to exit.
21. Test to see if your GC backups work now.
22. If they don't, repeat steps #15-21 again with different cMIOS files from the list each time (step #18 can be skipped from here on out.) until step #21 results in working GC backups.