With data-driven development one try to make it easy for non programmers to change the game. As much as possible should be able to change from the data that the program read in. Now content can then be added, modified and iterated on without being dependent on a programmer.
Read data from human readable formats
Make the games read it's content and what it should do and act from data files. By making them readable by humans they will be easy to modify by content creators. Example formats are XML or JSON.
Document the file formats
All file formats need to be documented so it easy to find out what the valid values is and when they change. Put documentation in the source code repository so it can be changed in one place and always be in sync.
Add editors to modify the files
With the format documented it is easy for anyone to make a editor.