WordPress.org

First of all, read http://wiki.buanzo.org/index.php?n=Main.Wp-enigform-authentication COMPLETELY. The plugin installation itself is VERY easy, but it's dependencies might not be that easy. Having said that:

1. Upload wp-enigform-authentication.php to the /wp-content/plugins/ directory on your blog's server.
2. Activate the plugin through the 'Plugins' menu in WordPress
3. IMMEDIATELY configure the plugin. Authentication plugins are tricky. Read http://wiki.buanzo.org/index.php?n=Main.Wp-enigform-authentication for details.
4. Modify your template as instructed in http://wiki.buanzo.org/index.php?n=Main.Wp-enigform-authentication
5. Did I mention you HAVE to read http://wiki.buanzo.org/index.php?n=Main.Wp-enigform-authentication ?