There is the guide to adding new content.
Additionally, in your cataclysm directory, navigate to the doc folder, and check out mapgen.md. There’s a lot of information in there.
There’s also been questions and comments here in the forum that you might find useful by using the ‘search’ function for specific questions. And, of course, you’re free to ask questions if you get stuck.
General advice? Copy and paste everything until you know what you’re doing, you’ll get fewer errors that way.