Keep an eye on the Campaign battle map - which can be accessed by typing either /cmap or go to the right drop down menu and look under maps/regional.
The areas that are solid dots with no sword over them are areas that do not have a Campaign battle in progress yet. You can stand in the city's and watch the notices that come across to get an idea where the next battle might begin.
If a battle is in progress already in an area, you just travel there and sign up with the C.A.
If you are in an area that has a fortification, you will be given notice that a battle has begun with a simple message on your screen that says "A Campaign Battle has begun...."
This will be followed by "The Campaign Battle has ended" when it is over.