Drive around in a tank that can jump, trying to shoot your opponents while dodging their bullets.
Use the arrow keys to nagivate to Join Game and press <Enter>.
Move down to Server and type play.onfoss.org and keep the port number at 5154. You can also set your Callsign here. This is how other players will see your name. Then move up to Connect and press <Enter>.
You should now be connected.