If you look at a scenario file, the format is pretty obvious, and there are some comments to help.
While I haven't tried it yet, you should be able to copy an existing scenario and replace the name of the map and the two orders of battle in the appropriate places. You can also change the season and weather.
The start time and duration of the battle is saved with the map in the map editor and is not specified in the scenario file. So if you want to change the start and end times of the battle, you need to edit a copy of the map.
Hook