Locate the Wurst Installer.jar (or .cmd for Windows) and run it using Java.
You can now use Wurst features on Minecraft Realms for both the standard 1.12 version and the 1.12 OF build. Download Wurst v6.21 MC1.12 OFFile: Wurst-Clien...
Grab the Wurst-Client v6.21 MC1.12 OF file from the official Wurst 6.21 update page . Locate the Wurst Installer