The first thing you need to do in the Editor is tell it to look for files in your mod’s folder. Go to the “Scenario” menu and choose “Scenario Properties”. There is a box called “Scenario Search Folders”. Here, type the name of your folder – “Mymod” or whatever. This means that the game will look for material in the Mymod folder. If it can’t find what it needs there, it will look in the main game folders. If it still can’t find it, it will crash.
You can tell it to look for multiple folders here. For example, you may be using units or other things that come in the Conquests scenarios. Say you’re using some graphic from the Rise of Rome scenario. You will need to enter into the box: “Mymod;..\conquests\Rise of Rome”. You can add as many more of these as you like. The game will always look first in the first folder specified, second in the second, and so on, before looking in the main game folders.
Mymod (and any other folders you use) must mimic the folder structure of the main game. Thus, the original CivIII folder is divided into Art and Text (and some other stuff that doesn’t concern us). “Text” contains several text files; “Art” is subdivided into various other folders, such as Units, Leaderheads, and so on. If you look in the PTW and the Conquests folders, you will see that they follow the same structure, but they don’t replicate everything. They contain only what is added or changed. For example, the PTW folder contains a Leaderheads folder that has material for the new LHs that came with PTW. The Conquests Units folder contains only the units that were added with Conquests. Mymod should be the same. If you add any units, you need to create a Units folder within Mymod\Art and put the graphics in there. If you don’t add any units, you don’t need a Units folder at all. The same for everything else.