For your custom cards, you have to put something that isn't already used by other cards, otherwise your cards won't load/work. The most important field is the card code, it's an unique number each card must have. Simply fill in all the fields you want and save the card into an existing. There are some programs that helps with the creation and management of those files, one of them is DataEditorX ( ).
The game looks for them in specific locations at launch to load all the cards. cdb to store all the cards you see ingame. First of all, a card database entry is needed.