STEP 1: as you work your way up to the battle scene, make a few notes to yourself. For example, you suddenly come up with a certain little detail. Well, write it down!
STEP 2: make an outline of what you want to happen in the battle (make it rough, so there's some wiggle room, which you'll need badly). For example: "Good guys charge. Bad guys form a line. Good guys strike. Bad guys feint retreat." Etc., etc., etc.
STEP 3: draw up a crude map of the battlefield and describe it right before the battle. Put in a few interesting details (a lone tree out in the valley) that you can use later (our hero pinned his foes to the tree with a spear). Put in the positions of all the armies and their units. REMEMBER: try and make the battlefield unique. Remember that BOTH SIDES want to get the TACTICAL ADVANTAGE. In other words, if there is a nearby Pass, have the armies try to sieze it. The army who doesn't gain entrance into the Pass will have to assault their now-entrenched enemie.
STEP 4: work out where your characters will be, who will do what, etc. For example: Main Character 1 goes on the left flank, Suporting Character 3 goes with the reinforcements, etc., etc.
NOTE: by the time you get to the battle, it will be so fleshed out you will barely have to make up things as you go. However, leave yourself some wiggle room so you can add a few "random" things or "new ideas" while in the middle of writing the Big Scene.
STEP 5: describe the moods of the armies. The feel in the air. The thoughts going through your characters' minds. Don't jump back and forth too much or it can get confusing; dedicate respectably large sections to each character so that it isn't too chaotic for the readers.
STEP 6: start writing the battle. Add in details, like Regimental Manuevering and such. As long as the framework for the battle is set, you'll do fine here.
STEP 7: put as much variety into your descriptions as possible. Instead of "both armies crashed into each other and loads of people began to die," perhaps you could write "The two armies met, the crash of their meeting echoeing across the plains. Blood stained the battlefield as the men fought for their lives, and for their country."
STEP 8: throw in a good kicker during the middle of the battle. For example: kill off one of your Readers' favorite Characters. Suddenly have hundreds of light cavaliers appear behind the General on the Hill and attack the army from behind. Stuff like that. REMEMBER: Each battle MUST be made different from the others, with different RISKS, REWARDS and ACTIONS taken in each.
NOTE: battles are very terrible ordeals. Expect huge amounts of Blood, Gore and Killing. If the two sides have very large armies on an open field of battle, expect THOUSANDS or even MILLIONS of soldiers to be slain. Large conflicts ARE NEVER, as a Rule of Thumb, PRETTY or even EASY. Almost all the time, victory is tough to achieve.
STEP 9: throw in the Climax of the battle, where everything hangs in Limbo. Will the good guys win? Will the bad guys win? Will the favorite characters survive? Will King Evilguy finally die?
STEP 10: assemble the Resolution: Decisive Victory, Costly Victory, Stalemate, Narrow Defeat or Crushing Defeat for the Good Guys.
STEP 11: describe the battlefield as the Characters go out to look for missing friends and allies or just to help the wounded. Describe in detail (but not too much detail) the terrible toll it has taken on one side or the other, and the varying amounts of carnage that have taken place.
STEP 12: wrap it up and put in the "Battle Epilogue" to tie up any loose ends. Remember at the end of Lord of the Rings: The Two Towers? When Gandalf, Theoden, Aragorn, Legolas and those other guys went up on top of that hill? Remember when Gandalf said: "The battle for Helms Deep is over. The battle for Middle Earth has begun." Well, that's the "Battle Epilogue". Use it wisely.
(((And there you have it! 12 steps to writing a good Battle! Don't overdo the detail unless you WANT an over Over-the-Top feel. But don't get me wrong! Make it as DETAILED AS YOU WANT IT. Remember that this is simply a 12 Step Help System.)))
ONE LAST NOTE: all military tactics and stuff like that should be put into context with the Time Period and Region of the place in which the battle takes place. For example: if it takes place in Ancient Greece, make sure you use tactics the Ancient Greeks would have used, such as phalanxes, shield walls and spear walls. Note that they NEVER used cavalry to breach infantry lines unless it was a last-ditch attempt. However, if it is Medieval, put in huge brawls with lots of armor, broadswords, claymores and battleaxes. IF IT IS MODERN, then put in Squad Combat, Squad-based Tactics, and even HUGE regimental movements. If "I'm naked, I'm drunk and I'm Irish" is still around, ask him about Modern battles. He's good at that sort of thing.
(((Hope this helps! I've been working on it for MONTHS!)))