get any controller, Nintendo switch, PS4, whatever is compatible with steam.
(you also have to use Bluetooth to connect it to your controller)

After you've gotten your controller, YOU gotta host the game, the player with the controller needs to host. in order to actually play the game with the controller you have to go in big picture mode, i know it sucks, but you have to do it.

now you have to invite your friend into a remote play session, and you have to press your "enter button" on your keyboard/controller then the game will start doing co-op

If you start playing, it take a bit to get used to the controller, but it gets easier the more you do it.
if one player dies, you all die.
theres only a limit of 2 people of co-op (at the time this was made)
Thats all i got.

if you have any questions, ask me in the comments.