Always back up any files you change or replace or you may damage your ability to play the game!
With that out of the way I'll see if I can't explain some of the basics. The best way to learn how the system works is to look at how units currently work, and then reverse engineer custom armies and weapons from there.
Unit creation has several dependencies.
Unit stats, Weapon stats, and unit appearances. For steam, the directory looks like:
Steam\steamapps\common\Warhammer 40,000 Armageddon\Graphics\Units
That location above is where the unit cards. When you look, you can see every unit graphically exists as a 256x256 PNG file posed in six directions. Be sure to pay attention to the numbered naming conventions! Once you make some custom unit art, this is where you'd put it.
For unit stats, you need to look in
Steam\steamapps\common\Warhammer 40,000 Armageddon\data
for a file called
unittypes.whdat
I strongly recommend opening this file with Microsoft Excel, because it will preserve the structure of the file and present it in a somewhat organized fashion. From here, you add, remove, or modify the game units, their stats, and their weaponry. Again, I highly recommend at just looking at this file and comparing it to units as they exist in game so you have a better understanding of what every field does and is responsible for.
Column Q through S is for weapons, here you assign weapons, but to set up weapons you need to dive into another file in the same directory.
weapons.whdat
Again, viewing and working with this file is best through Microsoft Excel. I highly recommend at just looking at this file and comparing it to units as they exist in game so you have a better understanding of what every field does and is responsible for.
There is also another step for weapon configuration that is weapon alignment, but that is a good deal more complex and the editor has functionality to help here (functionality currently offline though, but stay tuned for that!).
This is only a very basic explanation of what would be required to create entire custom units, weapons, and armies, but I hope it's a helpful start to get you pointed in the right direction.
