Upload ddf-signup.php to the wp-content/plugins directory of your Wordpress install.
Activate the plugin through the Plugins menu in Wordpress.
View the Secure Form - Signup page under Plugins to set options for presentation, spam handling, validation, and other options. The Secure Form - Signup page also provides statistics on the number of spam vs. total submissions.
To add a form to a page either create a template with the php function: <?php ddfs(); ?> or enter the shortcode [ddfs] in any post or page content.