There's no short answer to this. Take a look at the RECaptioningPluginInfo or the OSMF Captioning Plugin. Mainly you need to load a captioning plugin, attach the URI for a TTML file as metadata to your media resource before you turn it into a media element, listen for when TimelineMetadata is added to your media element, and then listen for the TimelineMetadataEvent.MARKER_TIME_REACHED event on the TimelineMetadata associated with your media element. The REOPS skin should have a closed caption field already in it for you to use. Using that is similar to using other skin elements.
Basically, the easiest is going to be to find a tutorial or sample using the OSMF Captioning Plugin to see how to load the plugin and how to attach the metadata. You should be able to then put that URI metadata into the REOPS config file if you're using that, or attach it manually.
Best of luck!
Nils