You will need the following before you begin:

• An image editor (SAI, Paint, GIMP, etc)
  
• Txt editor (Блокнот, Notepad++ etc)
  
• Installed basemod (You can check out how to do it here (Eng) (Rus)
  
• A separate folder for your own mod in BaseMods folder.

Inside the folder for your personal mod make another folder called GiftArtWork. This shall be the folder for the textures you'll be creating.
Here[www.nexusmods.com] is a mod as an example in case you need files to base your work on.
Once you've created a file put them in the said directory and rename them to:
YOURNAME_front - Is always displayed in front of the character.
YOURNAME_frontBack - Is always displayed behind the character.
YOURNAME_side - Is displayed whenever the sprite of the character turns.(Such a piercing attack). Displayed on top.
YOURNAME_sideBack - Is displayed whenever the sprite of the character turns.(Such a piercing attack). Displayed behind.
That's about it with this folder, lets move onto the next one.

Now make a StaticInfo folder, which shall contain stats and properties of the battle symbol.
Inside this folder make another one called GiftInfo.
AND THEN inside GiftInfo folder make a txt file called GiftInfo.
Open it up and copy paste the following text into it, edit it to your own liking.

Now lets talk about what each value means.

1. <GiftXmlRoot></GiftXmlRoot> - Serve as the start and the end of the code, make sure whatever text you will add/edit is in between these 2 lines at the start and at the end of the script.
   
2. <Gift ID="123456"></Gift> - This is an id value of your symbol. Make sure its unique and doesn't conflict with any other mods.
   
3. <Name>TestName</Name> - Name of the battle symbol
   
4. <Resource>custom_Name</Resource> - This is the directory of your battle symbol. It should be changed to custom_YOURNAME where YOURNAME is the name you've given to 4 texture files of a select gift, in the GiftArtWork folder.
   
5. <Position>Eye</Position> - The slot that the symbol shall occupy.You can use: None, Helmet, Mask, Nose, Mouth, Cheek, Eye, HairAccessory, Ear, Hood
   
6. <Count>1</Count> - This number shall be used later in the space of {0} in the localization step. Used to give a unique acquirement condition to the symbol.
   
7. <Stat></Stat> - Begining and the end of the stat list.
   
8. <HP>10</HP> - Amount of hp the symbol will give you. Can be negative.
   
9. <Break>10</Break> - Amount of stagger resistance the symbol will give you. Can be negative.
   
10. <Passive>20011</Passive> - Unique effect of the symbol

Time to make a third folder called Localize.
Make sure that the Localize folder has every relevant language folder inside it for you. If it won't have an English language inside, it wont work on an English setting, same applies for any other language. ("en" for english, "kr" for korean)
Inside each relevant language folder make a separate GiftTexts folder.
And make a txt file called _GiftTexts in the said folder.
Copypaste the following text inside and edit it if you so choose.

And now lets review each line in detail

1. <GiftTextRoot xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"></GiftTextRoot> - Beginning and the end of the script.
   
2. <GiftText ID="TestName"></GiftText> - The relevant id this localization file is attaching itself to. Make sure that the "TestName" is replaced with the same id as the gift ID in GiftInfo.txt.
   
3. <Desc>TestTest</Desc> - Description of your battle symbol. Is displayed in the main menu and also when you acquire it for the first time.
   
4. <Prefix>Cool</Prefix> - Every single battle symbol has a prefix and a postfix you can attach to your librarian's name. This is corresponding to the said prefix, as well as the first part of the displayed symbol name.
   
5. <Postfix>Guy</Postfix> - Postfix of the symbol, as well as the second part of its name.
   
6. <AcquireCondition>Test {0}</AcquireCondition> - Doesn't actually mean anything, you will have to either install an all ego mod on nexus or write a script in order to acquire your own symbol. This is just text. {0} - is the count number in GIftInfo.txt

And you're just about done.

Most info of the ingame symbols you can find in this directory: \Library Of Ruina\LibraryOfRuina_Data\Managed\BaseMod\StaticInfo\GiftInfo. Through it you'll be able to apply passive ingame effects. This ingame list should increase with introduction of new mechanics and generally game updates.
Localization of all the vanilla symbols are in this directory: \Library Of Ruina\LibraryOfRuina_Data\Managed\BaseMod\Localize\en\GiftTexts.

Once you made your mod either use this[www.nexusmods.com] mod that lets you acquire all ego gifts, or you're supposed to write a script for a unique way to acquire it.

Here's[www.nexusmods.com] also an example mod of a custom symbol, once again.

And here's original version of a guide by Pcix, in Russian.