Using an XML editor and the test concept you created in step 2 above, first verify that the document is still valid, which verifies that you didn't break anything in doing the integration of concept.dtd. If that succeeds, then add a <conbody>, a <p>, and verify that you can now add any of the XML domain elements within a paragraph.

If this succeeds, you're done. You have successfully defined a new domain and integrated it into a shell document type. Now all you have to do is repeat (that is copy) the shell integrations into the other shells you'll be using the domain with.

The next step, if needed, is to add domain-specific functionality to your DITA processors. For the XML domain, that means updating output processors to create the formatting effects for the different XML component mention elements.