Practice is your friend. I had it easy as I played a lot of STW before starting on MTW (which is more complex compared to STW).

Remember that you are operating an army, not a collection of units. There is a frontline, support, flanks (left and right) and a rear. Units operate together, supporting each other. Certain units are suited in different areas and tasks. There is no fixed way of forming up an army or determining the unit composition either. Personal taste, terrain and the opposing army comes into play. Also, the culture (catholic, orthodox or muslim) affects your unit types and choices, and hence the style of fighting.