the most important files for making a new division are in
(your mod's name)\GameData\Generated\Gameplay\Decks
(your mod's name)\GameData\Localisation
(your mod's name)\GameData\Gameplay\Decks

Steps to making a new division:

1. Go into (your mod's name)\GameData\Generated\Gameplay\Decks and open divisions.ndf
2. Choose one of the base divisions, eg.
Descriptor_Deck_Division_GR_20_Panzer_multi
3. use notepad++ findInFiles and search for all references
4. clone all the references and their associated text
5. rename all the cloned references

7. remove all the packs from your cloned Tdeckdescriptor in the Decks.ndf (this will make autofill in the deckmaker not work but who cares)

6. Create a new packs or choose pre made packs with the units you want to add to your division from packs.ndf
7. in Divisions.ndf replace the packs already there with the packs you chose in step 6
8. in Divisionrules.ndf add rules for all the units in the packs that you added
9. clone a divisioncostmatrix in divisioncostmatrix.ndf and give it a new name
10. replace the original costmatrix in divisions.ndf with the one you just made

11. in MODDING.csv create new text strings (clone the template and replace TOKEN with a custom string that is 1-10 characters long, and relpace REFTEXT with the text you want to show up in game
12. replace:
DivisionName = 'Div_20Pz'
DivisionNickname = 'SNGR20Pz'
DivisionDescription = 'DESGR20Pz'
with the tokens you just created.

--- IMPORTANT ---
If you clone anything with a GUID, eg. DescriptorId = GUID:{9c32d848-0825-4e85-8535-0dcd53e79193}, you need to replace the GUID with a new unique GUID
You can generate them by going to https://www.guidgenerator.com/online-guid-generator.aspx
There are GUIDs in Packs and Descriptor_Deck_Division.
Many other templates have them, but the above 2 are the only ones you need to know about to create a new division with existing units.