This tutorial answers the BIG question: How do I make maps for Rome: Total War. This task is pretty simple first, find the Rome: Total War program. Now, right click on it and go to properties. You should see The Target. Click on the end after .exe" than make one space and type -enable_editor. This enables the Battle Editor. Now launch the Rome: Total program you just edited. Now, you are at the game's main menu. Go to options, and at the bottom, you should see Battle Editor. Click on that and let the adventure begin!
This is very similar to Medieval II: Total War!