move your mouse over the land, press down and drag out, move to the land you want, then release mouse, you will see the blue dots will fly there. there's a number on the top of each land, that means this land can offer you that number of soldiers, so while you are taking over the land, your troops is becoming bigger. you can also find out how many soldiers you already have and how many soldiers are available to you by seeing the people shape panel on the top bar. 