Before the start of the battle, each opposing commander organises his/her army into units. The referee then works out how tough each unit is (i.e. its combat values).
Each commander then decides on the initial location/disposition of his/her units, based on the available terrain and who has the tactical advantage.
The battle then starts. The battle is played out in battle turns.
Each battle turn:
The battle continues until all units of one side have either been destroyed or have left the battlefield.
See also: an example of a battle.