Extract the contents of the MRPI you downloaded, and copy the extensions and mrpackages folders to your Kindle
Extracting KUAL
Extract the KUAL you downloaded and find the Update_KUALBooklet_*_install.bin file and copy it to your Kindle's mrpackages folder
Eject and unplug your Kindle
Running MRPI
On your Kindle, type ;log mrpi into the search bar and hit enter
Done
Now wait whilst KUAL is installed, your Kindle screen turn white and show some icons, after a while you will be returned to your library and see a KUAL book appear in it.
If you see a "Application Error" dialog, you can close it without worry - this is normal behaviour on some modern Kindles
Troubleshooting
Verify the location of all the folders and files on the Kindle
Try copying the Update_KUALBooklet_hotfix_*_install.bin file to the root of your Kindle when connected to your PC, and then go to Settings > Update Your Kindle, then resume from step 5