One of the best guides available for creating your own Open Access materials is Open Access Textbooks. Figuring out how to create and protect your intellectual property is one of the most important things you can do as an academic. This video from Open Access Textbooks explains some of the nuances of this process.
A number of best practices for remixing are available:
Modifying an Open Textbook: What You Need to Know, Open Textbook Network
OER Editing and Publishing Tools, University of Illinois
More information about localizing OERs: