Are you asking about how to write the battle scene itself, the political maneuvering that leads up to them, the reasons the battle takes place, or all of that and more?
If your plot arc is leading up to a battle, you should be including more and more information as you go, logically leading up to the battle as a conclusion-- large battles don't just start out of nowhere. Drop steady bits of information about whatever war-related technology your world has so that the reader won't be surprised to find out that both sides are fighting using magic dragons with lasers or something.
Knowing where everyone is, what their goals are, what the battlefield is like and all of that-- it's important, but largely depends on your world and the technology and magic they have.
I'm not big on maps and sketches myself, but then, I also don't generally attempt battle scenes on a large, grandiose scale. In that case, I would need a page or two of notes at the very least.