Download the Fabric installer
Visit the official Fabric website and download the universal installer (.jar).
Make sure Minecraft 1.21.11 is installed
Open the official launcher and start Minecraft 1.21.11 at least once before continuing.
Run the installer
Open the .jar, select version 1.21.11, check “Create profile,” and click Install.
Select your Fabric profile
In the launcher, select the “fabric-loader-1.21.11” profile before you play.