If creating a mod for the first time, see the detailed guide about creating a mod.

Sublime Text: Syntax highlighting for Sublime Text 3.

Here are text editors with CK2 language support:

Use comments starting with # character, to remember reasons for writing tricky stuff.

Vanilla uses 1 tab for indentation rather than spaces.

Indent properly to easily spot unclosed curly braced.

Use ANSI (Windows-1252) encoding for text files.

In case of a modding team, consider using a source control system like GitHub to act as backup and also manage collaboration.

Backup your work to avoid losing everything.

Use a proper merge tool, to merge between folders, and update modified vanilla files to a new vanilla patch.

Minimize overwrites of vanilla files by adding separate files and loading from folders whenever possible, to improve mod compatibility and maintenance.

Validate your mod with The Validator, it will save you a lot of pain and time troubleshooting crashes and bugs.

Use a good text editor to edit files and search into multiple files.

Create a mod for your modifications: create a personal mod even for small changes, and never modify directly game files in Steam CKII folder, as they may be overwritten without warning.

Ck2 agot sub mods