First, you will need to locate the sample template the game provides for you, mine is located in "D:\SteamLibrary\steamapps\common\Total Conflict Resistance\Workshop\TotalConflictWorkshop\Content\Data\SimpleModTemplates".
Once you have reached the proper folder, you will see these files.
The one you need is "SimpleMod_Faction". Open the file in notepad++ and you'll see this.
To start, pick the faction you want to change, type their name in the "RowName" line, and type the new faction name in the "Name" line. For the example, I picked Sambro and changed it to USA.
Then create a folder anywhere on your computer, and place the .json file inside this folder along with the new images we are adding. Make sure the image for your flag and leader are the right pixel count and named correctly. re-name them to, T_Flag and T_Leader, just like that.
Here is a reference sheet for image sizes.
Now we boot up the game. In the game you will see on the main menu, "workshop", click on this, then click on "Editor", then you will see this on your screen.
First, down at the bottom right you'll see "CREATE", click this it'll create a mod named MOD#01.
Now edit the fields appropriately. You can look at my picture and get a gist of what your doing.
For preview, click the blue folder and browse for your image of mod.
Then for content, click the blue folder next to it, browse for your mod folder you just created, select it, then the select "open folder". after that click apply, if done correctly it should make your mod and upload it to the workshop. Go ahead back out a page and go to subscriptions and check if your mod is there as well. If you see the mod in the subscriptions screen setup to the details you entered the mod was done correctly. I have noticed sometimes the mod may not show up and you may need to subscribe to your mod in the workshop to get it to appear but it doesn't happen often.