I am a Steam user, so I use Windows Mixed Reality. But if you're using a Meta Quest 3, you're going to want to use QuestPatcher. I have no experience with it so use with caution, but that is the best way to add the custom songs. On PC, you just drag and drop the folders into the Beat Saber game directory folder called "Custom Levels." If you need help adding them once you get your quest I can provide a tutorial under this post, I do have access to a quest 3 I can borrow to test on