My friend and I want to play some of the user created CO-OP created maps from this site. How do we set it up, or what steps do we follow in order to play the CO-OP maps together online (PC-PC).

Thank you.

NUMERO UNO: Make sure you both have the map(s) downloaded.
NUMERO ZWEI: Join a co-op game with your friend.
NUMERO THREE: Open console and type "changelevel <mapname>", except the quotes and <mapname> is the name of the map. If it's a series of maps, you should go to the first map.