In the web browser, go to the new GNU social install and register as a normal user. You'll use this user to post statuses and to friend users on other servers.

Now, close the site to new registrations then add the GNU social theme to the end of the config file generated by the installer:

echo "
$config['site']['closed'] = true;
require_once('plugins/GNUsocialTemplatePlugin.php');
\$tpl = new TemplatePlugin();
" >> /var/www/config.php

Now you've got your GNU social set up, you'll need to make it accessible to the world...