Hi there!
There wasn't a tutorial on how do make your own vanilla style faction symbols, so I decided to make one myself.
To make a new faction symbol in the same style as the original game has, is a very simple process, and only requiers some basic skill and knowledge with GIMP or photoshop. You can make one in a few minutes, but to get a nice result, it will take a bit longer.
So, before you read on, I will show you a few final results of the symbols I made, so you can see how your symbol will look after using this tutorial.

Faction symbol of VoC/Dutch, for Siam Total War mod.

Faction symbol of France, also for Siam Total War mod.

Faction symbol of Illian, The Wheel of Time Total War mod.

Faction Symbol of Mayene, also WoT Total War.
You will notice 2 differend sizes here. The first size, is for the campaign map. The second one has the size from symbol_48. It is always a good idea to use the larger size, since making a symbol smaller is no problem, but making it bigger will make it blury.
What do you need:
- Photoshop or GIMP
- A symbol or flag of the faction
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Part 1; Clean symbol
The first part is making a clean symbol. This will be the basic for every faction symbol you will be making using this tutorial.

Ok, to make a new symbol, from scratch. First we take an existing one, this can be a vanilla one, or perhaps this one. It really doesn't matter.

Now we select the inside of the shield. Use the tool I have selected, called free select tool. Not every single pixel on the side has to be selected, but make sure you do not select the edge of the shield.

I still had no green symbol, so I decided to make one. Fill the whole shield up with the colour you wish to use.

Now, we are going to add shine and shadows on it. Darker colours on the left, lighter on the right. For the right cornor on top, pick one close to the white, but make sure you can still see it is the colour used for the faction symbol.

Use the smudge tool to even the colours. Keep light colours right, dark left. You can use this one as an example. It will take a bit of trail to get it right.

Once you zoom out, this is the result. Now, on the previouse picture, you saw we still had some blue pixels on the sides. You can see it a tiny bit here, and it shouldn't be there.

So we zoom in again, to fix this. Select pencil, turn down the scale so you only draw 1 pixel at a time. Also, Opacity should be lowered. I suggest to set it at 50 or 60%.
Now, take the colour near the edge, for the right it is lighter as on the left. Click on each wrong coloured pixel, to make it turn green. If it doesn't work in 1 click, do another.
After this, you are done with a clean version of a faction symbol. I attache this one and a few others to this, for those who wish to use them.
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Part 2; Lines, multipel colours and symbols
So now you know how to make a clean version, as I like to call them. This is very simple.
Now, to continue, I will show you how I made to put symbols and multipel colours on it.
First, I had to decide what flag I was going to use as an example.
I found this flag:

Now it really shows the colours I have to use, but the symbol on it, is not good enough. That's why I use this symbol:

Big enough to be used. Size doesn't matter, as long it is larger as the faction symbol itself.
I picked a clean version symbol I already had made. I had the options for blue, red, or white. I picked red. It doesn't matter which one you pick.

So we open our clean red.

I will start drawing the white on it. First, select the part you wish to have a new colour.

Now we draw it in. Remember to go from dark to light (left to right)

Once again, use the smugde tool to get the result you like. It might be possible to add a little bit light or
dark to either side, but never do to much.

Now we do the same for the blue part.

Again smudge tool.

Open up the symbol we are going to add.

Most symbols you will find do not have a transparant back ground. Therefore, we need to click on
create new image: File > New
Be sure to set the background color to transparancy.

Now copy and past the symbol to the new one, and click on the fuzzy select tool. Click on the back ground, like shown in the picture.

Now we press delete. Delete any large white spot, and if needed, do some by hand with the free select tool. This will be needed to get some symbols out on a harder background as the one I have here.
There are still some white pixels on this, but those will disappear in the next step.

Resize the symbol. It is way to large for what we need. In GIMP, click on image > scale image. Mostly, it will take a few try outs before you get a nice looking size. I decided to go for 20 pixels here. Though, there is almost no symbol that will be used in a size over 100 pixels high.

Now make a new layer, and past the symbol on it. Move it around a bit if necessery.

To finish it, we will get ourself a new layer. On this one we will be using 2 colors; black and white. I recommend you use a brush or spray tool, but not a pencil.

Play a bit with the shades with the smudge tool. Sometimes it is better to keep the shade and highlight too just the symbol it self. You can also chose to give the faction symbol as a whole more dept and variation. See it as a finishing touch.
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I hope this helped someone. And for those who want, I attached some clean symbols you are free to use for your mod.
Resize for use as symbol_48.
- Yellow
- Red
- Blue
- Green
- Dutch flag
- VoC flag
For tutorials on other aspects of faction symbols or other styles, use these:
- Glow