I just installed it.
- In desktop mode, open itch.io, install the flatpack version of Cider. (Use the gear icon to select which installation.)
- Launch from itch.io, then run the flatpack file it opens, that will open up the program in Discover. Install that.
- Now search for Cider in your programs, and select add to Steam.
- Be sure to sign in to Apple Music while you are in desktop mode.
- You can use the Steam Grid DB decky plugin to add artwork and logos to it if you like keeping your library pretty.