Every battle begins with a phase where you have to decide the starting positions of your units. You can only place them inside the Deployment Zone, which is an area designated by a row of blue posts. The enemy has a deployment zone as well, surrounded by red posts. To move a unit within the zone, simply select it and place it in the desired position with the right mouse button. If you move a group of units this way, they will maintain their relative positions to each other.
You can also change the direction the units are facing by holding down the right mouse button and moving the cursor in the desired direction. When you are satisfied with the deployment of your army, click the Finish Deployment button to start the battle.