WordPress.org

1. Upload root-wp-cookie.php to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Delete your browser's cookies, or if your browser permits, delete only the old WordPress cookies from your site.

The third process logs you out of your WordPress admin page enabling you to log in again for the cookie to be placed on the root directory. It may be possible to log out the normal way (i.e., through the link from the admin page) but since the logout function has been changed to access a root-level cookie, it would probably be unreliable.