>>713877674
Here's a guide for people who want to play RetroRewind 6 on Dolphin but faced bugs when following the steps in the OP. This assumes you have a working version of Dolphin. (as in, you can launch the vanilla MKWii ISO without issues)

>1. Get the mod
https://drive.google.com/drive/folders/1nJp8g1F5YtTIABirO3wXjgzwkEd6qygZ

>2. Download WheelWizard
https://github.com/TeamWheelWizard/WheelWizard

>3.Point WheelWizard to the Dolphin executable location, Mario Kart Wii ISO location and Dolphin User data location in the settings

>4.Create your Mii and setup your profile in WheelWizard (if you don't want to be a nameless Player)

>5.Add the zip file of the RetroRewind 6 mod into "My Stuff" and install the mod with WheelWizard (the mod will be installed in appdata by default)

>6.???

>7.Launch RetroRewind via WheelWizard, play online and have fun with Mario Kart Wii

This worked for me yesterday but I skipped the mii creating option.

How to create/join anon rooms is explained in the OP