Just finished making my own first card, here's some basic beginner tips from somebody who'd never coded in a dynamic language before this point. If anything is wrong please correct me, otherwise if/when official mod support comes out, feel free to throw this text on in an FAQ/Help section somewhere.
Download Lua. I used https://github.com/rjpcomputing/luaforwindows to install on my windows machine.
To see code of base game cards:
1. Unzip a copy of the data folder into a new folder. Make sure this folder is in a separate location from any of the game's files (for example, I put this new folder in my PC's documents folder).
(By default, data will be a ~2,250,000 KB .zip file in Program Files/Epic Games/Griftlands/ )
2. In the unzipped folder, go to /scripts/content . Battle cards are defined in the attacks folder (basic actions are character nonspecific, Sal actions are Sal's cards, rook actions are rook's cards, etc).
Similarly, negotiation cards are under the negotiation folder.
My mod doesn't show up in the game!
Check the following list:
You've saved all your currently opened files in your editor.
Check for missing commas or '='.
In my personal experience, often times bad code will just cause your mod to fail entirely (though as of this point I'm only working in a single file). If you just recently added in some code, try commenting it out and restarting the game (CTRL + R will do this if you have debug mode enabled, getting an up-to-date version of your mod files).