add one line to wp-config.php
define( 'WP_ALLOW_MULTISITE', true );
login again – follow instructions
Note: Install WordPress in the root of your webpath if you want to use a sub-domain install. A sub-directory install creates a path-based network, even though it does not use file system directories.
add six more lines to wp-config.php
create the hidden .htaccess file containing the rewrite code