See the resources below for more information on authoring OER.
You might have to adapt an existing OER to fully meet the learning outcomes of your class. Both BCCampus OpenEd and OpenStax CNX provide resources to help you adapt an existing OER. Keep in mind, though, that some Creative Commons licenses require you to leave the work unchanged. Make sure you check the license on a resource before you start adapting it.
Some OER repositories have built-in authoring tools to help you adapt their OER.