Save and presto you're done :) Hit the TEST button and copy/paste the generated JSON in for example JSONLint to validate the output and yep we have valid JSON.
So this shows how easy it is to generate JSON these days. It has been different where we had to use the PL/JSON package (which was good too, but build-in is what I prefer).
And finally with Oracle Database 12c, and the JSON support straight in the database, consuming and querying JSON has been made so much easier, but that is for another post. If you can't wait, the link will bring you to the official 12c Database JSON doc.