As you've probably noticed (if you've tried to launch a mod from it's application) most mods will give you an error that says it could not find a valid kingdoms.exe application (or something along those lines). Easily one of the simplest solutions to this issue is to make your own "kingdoms.exe" application. Here's how:

1. Find your medieval 2 application. If you've downloaded it from steam, it should be located in C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War. In this folder, you should see an application that looks like this: https://gyazo.com/01210a3ec8bfe438bce37f56bd8fa493

2. Right click on "medieval2" (it might be labeled as "medieval2.exe"). Hit "copy" and create a duplicate of the file in the same folder.

3. Take your duplicate file and rename it to "kingdoms" (don't change the .exe if it's present) once you're done, you should see two applications, like this: https://gyazo.com/011fe552c370f6a2ebf154a8803f2a7e

4. Launch your mod.
Important note: For mods that require a Large Address Aware (Divide and Conquer, Third Age Reforged, etc. You should run the LAA on this application as well as the original medieval2 application

Of all of the methods for launching mods, this is probably the trickiest. Here's how it's done:

1. Go to your steam library, find "Medieval 2" right click on it and select properties. Like this[gyazo.com]

2. On the "Properties" tab select "SET LAUNCH OPTIONS"[gyazo.com]. When you do, it'll bring up a menu, type this into it:
--features.mod=mods/Third_Age_Reforged. (Change the "Third_Age_Reforged" to the name of the folder of whatever mod you're trying to launch.

4. Click "OK" then exit the tab.

5. The next time you launch medieval 2 normally, it should launch your mod with it. You will have to get rid of the launch options if you want to play normal medieval 2 again.

This is a simpler and more well-known strategy for getting mods to work. The steps are quite simple:

1. Find your mod folder, you should be able to find it under "C:\Program Files (x86)\Steam\steamapps\common\Medieval II Total War\mods".

2. Find one of the kingdoms expansions mod folders, these are "americas", "teutonic", "crusades" and "british_isles".

3. Rename one of the kingdoms expansions folders, as an example, get the "americas" folder and rename it to "realamericas" or "thisisamericas". It doesn't matter what you rename it to, as long as you can remember that that's the "americas" folder.

4. Find your mod folder. If you're launching Third Age Reforged it should look something like this[gyazo.com].

5. Rename your mod folder to "americas" (or whichever kingdoms folder you changed the name of).

6. Now, when you click "PLAY" on medieval 2, select the kingdoms expansion[gyazo.com] that your mod is now called. E.g: if I renamed "Third_Age_Reforged" to "americas" I would select the americas campaign. If I renamed it to "teutonic" I would select the teutonic campaign etc.

7. Now you can play your mod.

If you want to play the original kingdoms expansion again, you have to rename the mod that your currently using to what it was originally called (it doesn't matter if it's the exact name but just call it something you can keep track of), then take the renamed kingdoms expansion folder and rename it to it's original name. As an example, "realamericas" would become "americas".