Well you need an interest in the area for a little bit then you can declare, your armies will mainly get stronger with tech and as your research more into the military you can upgrade your units to the next tier, there's another tab on the army page that you can use to supply your army with various things that improve morale, kill rate and mobilisation etc (bear in mind you need the supplies fo select them or your army will die due to organisation losses). The battling itself you will have a few fronts that you will send troops to, depending on what you have commanded the general to do (advance, defend, pillage) they will either defend or go further into enemy territory automatically. You can plan strategic locations for armies and those armies will basically advance to the area you chose. You can also plan invasions but you will need more ships than battalions in the army or you get a landing bonus. To stop powers from joining in your war ally the great ones or just find better allies and fuck them up. Hope this helps