First, update the Apache configuration for the new site name:

• cd /etc/httpd/conf.d
• edit vhost-wordpress.conf
• Duplicate one of the existing site blocks.  They are almost all the same, aside from the hostname. The new one should be no different. (The only exception is for the server's default site.)
• Save.
• service restart httpd
• The URL should work, but take you to an "unconfigured" WP instance with registration disabled.

Create the new site in WordPress.

• Go to an existing site (e.g. http://usatlasfacility.org/) and log in to become an admin.
• From the My Sites menu at top, go to Network Admin / Sites
• At the top of the screen, click Add New.
  • Site Address: base name of new hostname (e.g. osg)
  • Site Title: name of site (e.g. Open Science Grid)
  • Admin email: email address of admin; try to match to existing user (e.g. rwg@hep.uchicago.edu)
  • Click Add site.

If the site is a usatlasfacility.org site, you're done. If it's in a different domain, you still need to set up domain mapping,

• Go to the new site's Dashboard
• Under Tools, click Domain Mapping
  • Add new domain: (e.g. osg.uchicago.edu)
  • Primary domain: yes or no, probably yes
  • Click add
• The new URL should now work.
• The site is ready to manage.  There may be other setup tasks, but these are generic wordpress tasks, not specific to our site.

-- DavidChampion - 26 Apr 2013