before you do anything else, it's absolutely vital you right click on Griftlands, click Properties, and then in the General tab, type --debug into the Launch options. This gives you all kinds of dev tools that makes modding easier. The most basic ways to use dev powers is ctrl + click on various things to bring up information, ctrl + ` to bring up console, ctrl + r to restart your game quickly, and ` in the main menu to bring up various testing modes.

As for your other questions, you will find it helpful to look at the code of other mods. The Shel's Adventure mod is a demonstration from the devs on how to implement new characters. You can find the mod files in Griftlands>Manage>Browse Local Files>click back in file path to workshop>content>folders contain mods by IDs. The base card files are under Griftlands>Manage>Browse Local Files>data_scripts>scripts>content> and both attack and negotiation. You can extract these 2 folders to your desktop for easier use. Look for cards that have elements you need, and then try to find which parts do what and use them for your own.

Griftlands modding is a lot to get into alone, you can try joining the unofficial discord (code GdrBVHF) which has a small but dedicated community of modders. We don't mind hand holding every once in a while, we all were there at one point. You can also DM me personally for help if you'd like, though keep in mind my modding knowledge is a bit limited. Playr 10#6710