For basic OER, such as slides or syllabi, simply add an open license to your resource and upload it to the web (make sure it is findable--see "How to Share OER" below).
To create book-length OER, consider using Schaffer's Manifold software / service.
For complex OER, such as textbooks, see Abbey Elder's OER workflow below:
Creating OER textbooks may count towards tenure and promotion as a pedagogical publication. Some schools, like the University of British Columbia, are incentivizing the creation of OER by recognizing the creation of OER in their tenure and promotion guide.