First, select the unit that you want to build by clicking on its picture in one of the panels on the right side of the screen. The selected type of unit appears in the large box on the left side of the screen, and the text provides details about the unit.
In the middle of this build panel are icons and numbers which tell you the cost of the unit. When you lack the necessary amount of a commodity it is shown with a red x superimposed. Above the costs you see two gold arrows and a small text box.
Click the right arrow to increase the number you are building, and the left arrow to decrease. The number you have ordered appears in the text box and near the unit picture on the right side of the screen. All units take one turn to build and are available for action next turn.
All military and naval units require a peasant laborer. Like workers in industry, each military or naval unit eats food each turn. If you build too many units too quickly, your industry will not have enough labor to continue to develop.