Thanks for downloading the Onion Knight mod. 

To install, go to where Final Fantasy III PR is installed on your computer.
In File Explorer, the address should look like this.
(ex. C:\Program Files (x86)\Steam\steamapps\common\FINAL FANTASY III PR)
Or you can go to your Steam Library, right click on FINAL FANTASY III > Properties > Installed Files > Browse

Next replace the original UnityPlayer.dll with the file of the same name found in the mod. 

Next go to FINAL FANTASY III_Data > StreamingAssets > aa > StandaloneWindows64
This is where all the bundle files are.

In the mod folder under the folder Bundles, click on the character folder and take the bundle files in there are drag & drop them onto the StandaloneWindows64 folder, replace the files.

Launch the game, and hopefully everything should install correctly. Enjoy!