Regardless of whether you create an OER from scratch or reuse an existing resource, there are a few things you need to consider in advance...
Take this short course from the Open University to understand more.
See the National Forum on Teaching and Learning recommendations on this topic Learn how to adapt Open Textbooks (National Forum on Teaching and Learning) Work through the process of developing and licensing an OER with the Open eLearning Content Observatory Services.
Explore the links and resources on Wikiversity's OER course.