If you haven't already, go to the Beat Saber Legacy Group Discord here:
Join the Beat Saber Legacy Group Discord Server!
and download the latest downgrader app in this channel:
Next, you'll want to verify that you have the latest version of Beat Saber installed on your quest (the latest is currently version 1.15 . This is important. If you don't have this latest version, the patcher will fail.
Next, open Sidequest, and click on Applications. If you don't already have Sidequest installed, what are you waiting for! Install that first here
Once you clicked on the Applications button in Sidequest, search for Beat, then click on the gear icon for your Beat Saber game (not the demo)
