It's dead simple - all you have to do is choose a battlefield and then use war tactics to shoot your opponent's warsheep until they're all gone.
There are three battlefields to choose from and four kinds of warsheep with different weapons to fight with (Rocket Launcher, SUV Machine Gun, Tank, and Air Strike). One warsheep is selected randomly each turn. You can shoot without moving your warsheep or move it into place and then take the shot. When it's your opponent's turn to shoot, you'll get a full view of the battlefield, so you can plan your war tactics.
To invite a friend to a game, right-click on his/her name on your Contact List choose Start and select the game from the list.