How to configure RDF on fields?
The Drupal built in RDF module works automatically and cannot be directly influenced from the user interface. There is however the Schema.org configuration tool (RDF UI) module. 


  The module enables you to specify mappings between content types and
  fields with types and properties of Schema.org and build Content types
  based on schema.org. Through the rdf module, specified mappings will
  be embedded in HTML as RDFa, to provide semantically rich data.


The video on the module page explains the process quite well.