In antiquity officers would be trained to look for flag signals, and audio cues like different signals from horns. These techniques were used for a very long time. For more detailed orders, runners were often used (they might be on horseback depending on the era and situation). In some cases the supreme commander would have to give lots of independence to his generals because the battle front was so big. Napoleon was famous for this kind of thinking, and it's still a major part of the orders writing format contemporary military officers use called battle procedure. The simple idea is that you know your commander's intent 2 levels up. So if you're a Lt. in the Army you know the intention of the Brigade commander. If you're a Private you know the intention of your Lt. The idea here is that every soldier knows what their conceptual big picture objective is. Especially when the enemy reacts against you and plans need to change quickly.