While I was trying to setup my cosmic session I wanted to use Cosmic Greeter as my Display manager because I wanted to test it out and give my own feedback/report any bugs I could find.
However I didn't find any guide on how to do it plus there's little to no clue on how to set it up and it took me 2 days to figure it out.
That's why I write this quick guide.
Cosmic greeter by itself doesn't work it needs a Compositor (in this case Cosmic-Comp (obviously)) to actually run.