You must select the recruitment button in the side bar shortcut of a town or hero. Then you need to select units (from units unlocked in the town, based on its development), and the quantity. For each type of unit, you have a delay to recruit one unit, and a cost (gold, and rare resources for powerful units). Finally, you confirm your choice with the OK button. Your units then start being recruited, and the end of the recruitment order is shown on the timeline.