Here is how to do it:
- Start the game via Steam.
- Click either Campaign or Versus.
- Tick the host game -box. If you want to beat some unknown people leave the box "Friends only" unticked. Campaign can only played with people that are your Steam friends.
- In Campaign-menu, choose your character first!
- Click on the type of game you want to play (1vs1 or Rumble in Versus-menu, or Multiplayer in Campaign menu).
Huzzah! You should be hosting a game and your friends/unknown people should find it.
If you want to find a server, just leave the host box unticked.
Occasionally Steam's NAT-punching doesn't work like a charm, which means you need to forward some ports on your home router if you want to host. You should always be able to find a game if someone is hosting one! This affects about 10-15% of users. Unfortunately, there is no way to tell if it works, other than by having a friend check out if they see your server.
Developers are known to enjoy beating people in 1vs1 and will be hosting a few games each day, see if you can catch them online!