I recommend this video by Commet. After you get the folder get back to here.
This section will show where do the files have to go...
Drop the courses in [your mod folder]/DATA/files/Race/Course. If it asks you to replace the files. Make sure to replace them
A bit harder then courses. I recommend this guide by Commet (replace the files)
Once you are done with installing the mods. Left click while holding shift then click PowerShell windows in your roms folder. (ex: roms/wii) Not in your mod folder
Then if you watched the previous video. (make sure that wit is installed) simply do
wit copy .\foldernamegoeshere\ mod.iso
(you can replace mod.iso with anything else. But it needs to have .iso (this will only show in explorer, on dolphin only the name you have given the mod in part 1)