First, open FOG2 and select “Editor” from the main menu. Click on the “New” button, and a New Campaign window pops up. Complete it as per the following screenshot, and click on the “COPY OF: <NONE>” button.

The campaign names that you put in the two fields can be anything you want. You must have no spaces in the top field, but you can have spaces in the bottom. The top field is the name of the folder that you will create in your My Documents/My Games/FieldOfGlory2/CAMPAIGNS folder. The bottom field is the name of the Scenario (Campaign) module that will display in the game menu. Unfortunately, when copying a vanilla scenario module, your new module will ignore the name you put in the field, and use the vanilla name instead. I will tell you how to correct this at the end of this guide.
After you have clicked the “COPY OF: <NONE>” button, the following menu pops up:

It is a list of all the currently available vanilla campaign folders. Note that any user created scenario folders do not appear. Also note that the list includes all the Historical Campaign folders, this is potentially confusing, as the battles in campaigns are randomly generated and you cannot edit these using the editor.
Now, choose the folder that contains the scenario or scenarios that you want to amend. All the historical vanilla single player scenarios are in the numbered folders ie: 1Immortal_Fire and 2Rise_of_Rome. It is reasonable to assume that future DLC scenarios will be in numbered folders 3, 4 etc. The Multiplayer scenarios are in seperate folders are at the bottom of the list, ie: MULTIPLAYER/MPImmortal_Fire, etc.
If we want to edit the Gaugamela scenario, we would select CAMPAIGNS/1Immortal_Fire, then click on “PROCEED”:

This will take you back to the NEW CAMPAIGN menu, and you will notice that the “COPY OF: <NONE>” button has been renamed “COPY OF: CAMPAIGNS/1Immortal_Fire”. Click on “PROCEED”:

The Editor has now created a copy of the entire vanilla 1Immortal_Fire folder in your My Documents/My Games/FieldOfGlory2/CAMPAIGNS folder.
After a few seconds the main Editor screen will appear, with a “Scenario Editor” menu showing a list of all the scenarios in 1Immortal_Fire:

You will notice that there are usually two versions of each battle. Most battles are playable from both sides, so there is a separate scenario for each side. The player side of each scenario is identified by the suffix at the end of the scenario name. So Gaugamela_M is the Macedonian player version of the battle, Gaugamela_P the Persian player version.
So select the scenario that you want to edit and press “Select”. The scenario will load in the Editor and is ready for editing.
Please refer to the other parts of this scenario guide to see exactly what you do to edit the scenario to your requirements.
Most people will probably want to add or edit units to the battle, so check out Part 2 (Placing the Armies) of the guide. If you want to add units to a single player scenario, you should also add them to an AI Team (see Part 4). If the new units are going on the AI side, you might want to give them orders as well (again, see Part 4), otherwise they will probably just advance at whatever enemy is nearest.
I would suggest that you check the existing AI teams, to see if you can add the new units to one of those. That would save you the trouble of assigning orders to your new team. For Gaugamela for instance, if you were adding extra levy or irregular foot to the Persians you could add them to the existing AI team 4, see this screenshot:

You might also want to give the new units a general, so see Part 7 of the guide.
After you have completed your editing, save the scenario, you can just use the existing vanilla scenario name, because as this scenario is in a totally different folder there will be no conflict.
At the moment your edited scenario module is called exactly the same as the vanilla one, making identifying which is which in the game menu a bit of a challenge! So now we need to correct that. Go to your new scenario folder, it will be in My Documents/My Games/FieldOfGlory2/CAMPAIGNS. Open your scenario folder and then open the Text1.txt:

In the Text1.txt at or near the top, you will see a line called IDS_CAMPAIGN_NAME, followed by the name of the Scenario Module. Just rewrite that name to what you want your scenario module to be called. Save the Text1.txt and close it. In our example I have renamed it Immortal Fire Mod, by changing the line to this:
IDS_CAMPAIGN_NAME,"Immortal Fire Mod",
For some reason, even though the Editor has copied most of the original vanilla files into your new scenario folder, it creates a brand new blank Campaign.txt, which is not very helpful and will result in your new scenarios not showing all the desired information in the game’s scenario selection menus. So while you have your new scenario folder open, go to the vanilla campaign folder in the game’s main installation folder. You will find it here: C/Program Files/Slitherine/Field of Glory II/Campaigns. Open the original campaign folder that matches your new edited module, copy the Campaign.txt that you find inside, and paste it into your new scenario folder, overwriting the new Campaign.txt that is there.
When you now go into the game’s Epic Battles option, you will see your new module displayed in the list. Select it, then select the scenario you want to play.