5. Other Setup Steps
1. Create a blogs.dir directory under /wp-content/
This directory is used to stored uploaded media for your additional sites and
must be writable by the web server. They should be CHOWNed and
CHMODed the same as your wp-content folder.
2. Add the extra lines provided to your wp-config.php file
These lines are dynamically generated for you based on your configuration.
Put these lines right under the WP_ALLOW_MULTISITE line, mostly for
readability.
3. Replace the content of your .htaccess file with the following
(If there isn't one, then create it.)
These lines are dynamically generated for you based on your configuration.
4. Once the above steps are completed and the new wp-config.php & .htaccess
files are saved, click "Return to Dashboard" to refresh your dashboard.
http://codex.wordpress.org/Create_A_Network
14. Child Themes
A WordPress child theme is a theme that inherits the
functionality of another theme, called the parent theme,
and allows you to modify, or add to, the functionality of
that parent theme.
http://codex.wordpress.org/Child_Themes