Creating a child portal is just like creating any other portal. There is a radio button to click that specifies a child or parent portal.
In the case of the site jncraig.com, a child portal's URL will be jncraig.com/newportal, assuming that "newportal" is the name of the new portal. Then, you browse to it as
http://jncraig.com/newportal.If you want to later have this site use the URL newportal.com, you'd have to have your DNS point newportal.com to the same IP address as your existing site. In my case, I'd have the DNS set up so that newportal.com points to the same place as jncraig.com. And, in portalalias for the site, I'd insert newportal.com in addition to jncraig.com/newportal. Then either address will get to the site.