Your guild has to have built up the guild hall to level three. Your host or VH can do this when contributions reach a certain level. After the guild reaches level three the guidl warfare icon appears on your screen. The host or one of the VH's will choose one of the cities available for your guild to fight for - then you can click to join the battle. If you click to join you do NOT have to be online your character is set to defend somewhere. If you are online you become one of the attackers with the rest of your group and move through several stages to help your guild win or defend a city.

No, this is not comprehensive and probably not clear, but it gives you an idea