In order to start a Guild War, then you first must be the Guild Master of a Guild. Once you have found another Guild which have agreed and are willing to have a Guild War with you, then you must offer the opposing Guild the Guild War. There are two ways in which you can do this. You can either target the opposing Guilds Guild Master, right click their name and click Declare Guild Battle, or you can open the Party/Guild menu, and click on Apply for Guild Way from there.

Once you have applied for a Guild War with another Guild, you will be bought to a screen in where you must enter the amount of deposit which you want to put down, and the length of time which you want the Guild Battle to last.
Deposit: This works on multiples of 10,000's. So, what ever amount you put down, it will be multiplied by 10,000. For example, if you put down 10, then you will be depositing 100,000 gold. The limit in which you can deposit is 100 (1,000,000 gold).

~~Still in Progress~~