Head over to the Actions tab and click on the latest run (at the top).
Then, scroll down to the Artifacts section and download the artifact.zip file,
in it should be a file called legacyfix-2.0.jar.
Note: Downloading artifacts requires a GitHub account, though you can use nightly.link to download the latest build without one. By clicking here you can download the latest build via nightly.link.
Important
Make sure Enable online fixes is disabled in the instance settings, as LegacyFix does not work with it.
You can do this by editing the instance, go to Settings -> Miscellaneous, and then you will see a section called Legacy settings, and there you can turn off Enable online fixes (experimental).
Create a new instance or edit an existing one, then go to the Version tab.
On the sidebar, click the Add Agents button:
And point the launcher to the legacyfix-2.0.jar file you downloaded earlier:
A new entry should appear in the list:
Prism Launcher defaults to outdated Java 8u51 (from July 2015) on Windows, 8u202 (from January 2019) on Linux, and 8u74 (from February 2016) on Intel macOS.
If you're using Windows 10/11 with Intel HD Graphics and you know that you need to be using Java 8u51 for Minecraft to run, read this document.
Otherwise, follow these steps to get a recent build of Java 8:
Edit your instance, go to the Settings tab, then select the "Java installation" checkbox:
Next, click on the Download Java button, pick Adoptium or Azul Zulu (we recommend Azul Zulu), and select the most recent release of Java 8:
Then click on Download and wait for it to finish.
Next, click the Auto-detect button and select the Java installation you've just downloaded and click Ok:
And it's done! You're ready to play Minecraft with LegacyFix.