download the game for windows/mac here
on gnu/linux, install the game using your package manager: apt, yay (AUR), dnf, etc.
when you open the game go to Multiplayer, then Specify, and type oa.cult9.xyz in the Address bar.
leave the port number default.
if you are stuck on the Awaiting Challenge screen, nobody else is online. invite a friend or two!
that's it, have fun :)