Unzip the downloaded folder. Do not run the installer from inside the zip file.
Wurst Client v6.21 for Minecraft 1.12: Radar, Realms Support, and OptiFine! Download Wurst v6.21 MC1.12 OFFile: Wurst-Clien...
You can now use Wurst features on Minecraft Realms for both the standard 1.12 version and the 1.12 OF build. Unzip the downloaded folder
Click the "Install" button. You should receive a success message. Download Wurst v6.21 MC1.12 OFFile: Wurst-Clien...