Curate the materials that best serve your purpose.
As you saw in Step One, many resources are available for building your class. But it might take some time and effort to find the ones that best serve your purpose.
In a textbook, the editors have already ordered all of the readings and lessons in a compelling manner that scaffolds the materials so that more basic concepts provide the foundation for more advanced ones. If you are remixing lessons from different OER, make sure that you are following this principle so student learning can build upon itself. Furthermore, be sure to provide clear transitions and exposition to make sure students understand important concepts they will encounter in different resources.
For help in this endeavor, you may reach out to the master educators in CAST, The Committee for the Art and Science of Teaching. CAST is an interdisciplinary committee of faculty members and professional staff commissioned to create and implement professional development opportunities which aim to enhance and highlight the teaching, learning and technology.