The Mod txt file or modified era file is the foundation of all mods, this is a very simple how to on creating a basic mod txt file.
I will be using the Early txt file as an example. You may choose any era file to start your mod.
Go to My ComputerC:/Program Files/Total War/Medieval Total War/Campmap/Startpos.
Double click on the Early txt file to open it.
In the top left corner, click on File to open the drop down list of options.Choose Save as and select a new file name.
You can use any name for this file I will use Mod.txt
Enter your txt file name and click on Save.
Now we have a basic Mod txt file to start working in.
Now scroll down the file to this heading.

//========================================
//campaign name: can either be text or a
//label. Set predefined accordingly
//========================================
cn_predefined::true
cn_title::"Early Period Title"
cn_short_title::"Early Period Short Title"

//========================================
//campaign description: can either be text
//or a label. set predefined accordingly
//========================================
cd_predefined::true
cd_line::"Era_Description_EARLY"
cd_predefined::true

//========================================
//hidden: set to true or false
//true means it won't show up on custom battle screens
//========================================
cd_hidden::false

For our Mod txt file to function we must make some changes.
Change cd_predefined:: true to cd_predefined::false
The cn_title is the mod name and will appear on the period select screen. you may choose any title you wish.
cn_title:: "Mod" (Be sure to use the quotation marks)
cn_short_title::"Mod" {Be sure to use the quotation marks)
The next lines define the description of your campaign, when you choose your mod from the period select, the description you enter here will show at the bottom of the screen.
Change cd_predefined::true to cd_predefined::false
cd_line::"My_new_Mod" what you enter here will be on the era background descriptions.
cd_predefined:: false (note: this line is for version 1.0 and 1.1 it is not used in 2.0
cd_hidden:: false the header explains that this is used to make your mod txt appear on the custom battle menu or not.
After you have made these changes double check everything then save. Now start MTW and when you reach the period select screen you will see your Mod as one of the choices.
Note: sometimes the Mod title will be at the bottom of the list be sure to scroll through the choices to make sure your title is there. If you followed these steps you should now have a working Mod txt file , from which you will be able to build your Mod. LOS