how to plan a battle ?

Michael's right about perspective. But the main difficulty I had when I started writing fight scenes is the actual combat part is relatively short lived, in most cases.

It depends on your period, but battles are mostly a confusion of noise and smoke. Before the 1940s and reliable radio, hardly anyone in the ranks (including most officers) knew exactly what was happening at any one time. Everyone's probably hungry, unless they were lucky enough to have breakfast first, thirsty, depending on the weather, their clothing, or the type of firearm they're using, and frightened. And much the same goes for their horses, before unfeeling tanks came along.

Some writers use what I'd consider an excess of gore when describing battles - anatomical rather than emotional detail. Getting the balance right is important, and much of that depends on your target audience, I guess - same with any sexual content.

If you're using a real historical battle it's useful to be familiar with what actually happened, so you can insert your characters seamlessly, without altering 'accepted' facts. Unless you're writing alternate history, of course.

Apart from that, your imagination should see you through :)

Suppose this depends on what kind of perspective you will use, an individual or many people, or the 'God veiw.'

I kind of avoided the issue to begin with, I wrote very little about a planned battle as I wrote from the perspective of my main character who was not involved. This allowed my story to continue to flow without getting bogged down in something that, for me anyway, was not most important aspect of my story.

I did come back to the battle, but once the rest of the story was more clear.

Maps (real/your own), documentaries & poetry. Narrow perspective by what era/level of tech is suitable.

I must thank you as this prompted me to look into Voices of the Great War and remember how much I love The'Falling Leaves' by Margaret Postgate Cole.