This post is a quick summary of how to enable multisite on your WordPress install. You should be starting from a fresh WordPress install, anything else can cause issues.
For a complete description of multisite and setting up multisite see the great reference created by Mika Epstein & Andrea Rennick:
1) Enable Multisite in WordPress
In your wp-config.php file, right before the line /* That’s all, stop editing! Happy blogging. */ add:
2) Click on the “Tools” menu item in the dashboard.
You should now see a new item “Network Setup”.
Click on “Network Setup”, and complete the Create a Network of WordPress Sites page. If you are not sure about the options, see the ebook above.
3) You will get a page that looks something like this after clicking install. Follow the 3 steps on the page to complete the setup. NOTE!!! Your settings will be different from the ones shown below.
Login to your site and you now have a network of sites or “multisite”.
There are many config settings that should be checked before using your site, see the ebook for details if you are not familiar with configuring a network of sites.