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.
Create a new instance or edit an existing one, then go to the Version tab.
On the sidebar, click Open .minecraft button:
And move the legacyfix-2.0.jar file you downloaded earlier into the instance directory:
Next, go to the Settings tab.
Enable Java arguments, and add -javaagent:legacyfix-2.0.jar to it:
Additionally add -Djava.util.Arrays.useLegacyMergeSort=true if you want to play versions before Beta 1.6:
And it's done! You're ready to play Minecraft with LegacyFix.