Ordering a new unit is simple--just click on the right arrow under the picture of the unit to be constructed. If you have the required money and commodities you will see a number 1 appear under the unit's picture. If you don't have the commodities in stock, the items you lack will be shown in red text.
As with other orders, you are free to change your mind. Click the other (left) arrow under the unit to cancel the order and restore the items to the stockpile. The new units will be built when you end the turn and they will appear on the map next turn.
