Debug effectively by enabling Debug mode.

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 braces.

Use UTF-8-BOM for localisation files (.yml).

Consider using a source control system like Git and a collaborative forge like GitHub to manage team collaboration, or just make a copy of the file somewhere else.

Backup your work to avoid losing everything.

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

So choose a name, no one else will ever use, like the name of your mod. (Your files can have any name, all files in the folder will be loaded by the game.

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

Use a good text editor (like Notepad++, Sublime Text, or Visual Studio Code) to edit files and search into multiple files.

Never modify game files: use a mod even for small changes, and never modify directly game files in Steam Hearts of Iron 4 folder, as your changes may be undone without warning.

Heart of iron 4 mods