Use 'dictate to' when showing someone's in control or power over others, often suggesting lack of freedom or choice for those being directed. The monarch would dictate to his courtiers every aspect of the court's etiquette.
Pro Tip 2/3
Negative Implication
'Dictate to' can imply the speaker is being too controlling or authoritarian, which might be seen negatively. Employees dislike how the new manager dictates to them all the time.
Pro Tip 3/3
Hierarchy Implied
It suggests a hierarchy where the person dictating is at a higher level than those being dictated to. The general would often dictate to his soldiers the strategies for the upcoming battles.