Before each combat you have a possibility to settle troops on the battlefield. The troops are displayed on a panel of seven cells. To start a combat, you are supposed to settle all your troops, those yet unsettled will be marked in grey. Every player has 3 minutes for combat settlement, if the "Start combat" isn't clicked within that time, the combat will start automatically and your troops will engage into fight unsettled.
To settle troops, simply drag them from the panel to the battlefield. You can split a stack by by dragging a creature icon from one panel cell to another while holding the Shift button. You will not see the settlement of your adversary, even if the troops are already settled, until the beginning of combat. However, neutral armies are exception on this rule; they are visible during the settlement.
You can also split a stack using a special button on the creature icon: