The editor is very easy to use. That will be where you set your scenario up. Start with the date for the battle and then onto the map. I recommend using the extended map editor, off of the main menu, it's handy "undo" will be the most important feature you will ever need, as laying roads sometimes take on a mind of its own
. You can generate a map with it and then tweek it. Save it and then, back to the scenario builder where you now load your map, set your date and now choose opponents and purchase the troops. The rest will be history!
