Convert a WordPress Single Site to Multisite

Quickly convert a single WordPress site to a multisite install:

1. In wp-config.php add the following line:

   define('WP_ALLOW_MULTISITE', true);

2. Login to the site
3. Go to Tools > Network Setup
4. Fill in the requested information and click Install
5. Copy and paste the items from the first box into your wp-config.php
6. Copy and paste the items from the second box into your .htaccess (replace what was there)
7. Logout
8. Log back in

Done!