>>312598049
You need:

https://github.com/Assistant/ModAssistant

and

https://github.com/bsmg/BeatSaber-IPA-Reloaded/releases
(BSIPA-x64-Net4.zip)

Usually you only need mod assistant, but it's bugged currently and can't install BSIPA.

For custom songs, you use this command line interface:

https://cmlsc.itch.io/beat-saber-song-downloader/purchase

Donation is optional.