I can recommend Maptool
for making maps. Half an hour (or less) and you'll be able to produce something that looks pretty good. You've also got the option of adding units to the battlefield as you need them, including PCs. This isn't as interactive as a tabletop with smartly painted figures but it's not a bad option. It's certainly less prone to misinterpretation than just describing things.
Anyway, I agree with Escheton. It's better to just start and wing it a little. Remember that all you need to work out is what our characters know and maybe a round or two ahead. We honestly won't be able to tell the difference between this and a battle that you've worked out in detail for weeks. Well, not much anyway.