For the export and import of textures:
PackFileManager[sourceforge.net] or the Rusted PackFile Manager[github.com]

For editing the texture files:
PaintNET[www.getpaint.net]


Note: There are several other programs you can use. These are the ones I utilize. For editing the DDS files I am using Photoshop, but you can also take PaintNET for it.

Location of the variants_dds.pack:
The texture files are in the variants_dds.pack in the case of Three Kingdoms.
You will find this pack file in the data folder of the game.
(default path: C:\Program Files (x86)\Steam\steamapps\common\Total War THREE KINGDOMS\data)

Open the variants_dds.pack:
If you found it, you have to right click the pack file and select 'open with'.
There you chose the program you installed for opening pack files: PFM or RPFM

Finding and extracting the right DDS file:
Now you need to know, which texture you want to edit. If you want to edit a character or a unit, you have to take a look into their variantdefinition. You can find those in the fast.pack and variants.pack files.

In there you need to expand the variantmeshes and the following variantdefinitions folder.
In the variantdefinitions folder you will find every variantdefinition of every unit in the game.
When you found the right definition, you have to open it as a text via right click. (RPFM left click)

There you have given the paths of every variantmodel the character/unit is using.
(weapons do have their own variantdefinition file, look into the 'set folders')

Now you need to open the variants.pack file in a second window of PFM/RPFM and expand the variantmeshes and the following variantmodels folder.
In it there are six another folders:

• character ~ textures of body parts and character models
  
• examples ~ unimportant
  
• fauna ~ textures of wild animals
  
• flags ~ flag textures
  
• mount ~ horse and saddle textures
  
• props ~ weapon and armor textures

There you will follow the path of the respective variantmodel, which you took from the variantdefinition. Extract the tex folder to a folder of your choice via right click.

Opening the dds file:
Now you need to go to the 'tex' folder, you extracted before.
In there are mostly three different DDS files. To edit the textures you need to edit the '..._base_colour.dds'.

To open it, you have to change the DDS to a DDS2 file. Simply rename it with a '2' at the end.
If you have done that, you need to use PaintNET to open the DDS2 file.
You are allowed to edit it directly in PaintNET.

When you finished editing you need to save it with the same name (but without the '2') as a DDS file.

Note: If you want to use a different program for editing, you need to save it as a png file
and open it in the other editing program afterwards.

Creating your own mod:
Note: If you already created a pack file for your own mod please ignore this step.

Now you will create your own pack file.
For that you open PFM (RPFM) and go on 'file' ('PackFile') in the left corner and chose
'New' ('New PackFile').

Importing the textures:
After the previous step you right click on the 'Untitled.pack' ('unknown.pack') and chose
'add folder' ('add' -> 'directory'). Now you will select the variantmeshes folder, where your 'tex' folder is in. This is necessary, otherwise the paths are not correct and your mod won't work.

There you have it! Check the mod type through clicking on the file menu in the top left corner.
It needs to be on 'mod' to work correctly.
Save it with a name of your choice into your data folder and upload it to the workshop.

Some units and generals are using the same armor/weapon models. Therefore they share the same textures. If you want to create independent textures for each unit you need to copy every file (models, textures, etc...) related to the armor part/weapon and rename it. After that you have to edit every file to have the correct paths with your renamed files. I will add instructions on creating independent textures if several of you need help with it.

I hope that this guide will help you. As I mentioned before, if you have any questions or suggestions, please write them in the comment section. I maybe will add some screenshots later for illustration.