Know the Map
After you select a province, you enter the section overview. This is where you set up your defenses and take a peek inside past battle logs. Here, each member has the chance to launch his or her individual attack, so communicate with your friends and make sure everyone’s on board before declaring war on an opposing guild. That’ll give you a bigger chance of taking down your target.
Choose a Sector
Attack & Defend
Once you're victorious, your siege army moves in and is ready to defend your newly claimed territory. A recently conquered sector will be protected from incoming attacks until the end of the day. Then, every other guild has the opportunity to charge into your sector.
Keep in mind that guild battles are currently only available on the Forge of Empires test server before they will eventually find their way to the proper browser game world. Some things are bound to change until then.
Source: Official forums