Put subscribe-to-comments.php into [wordpress_dir]/wp-content/plugins/
Go into the WordPress admin interface and activate the plugin
Optional: if your WordPress theme doesn't have the comment_form hook, or if you would like to manually determine where in your comments form the subscribe checkbox appears, enter this where you would like it: <?php show_subscription_checkbox(); ?>
Optional: If you would like to enable users to subscribe to comments without having to first leave a comment, place this somewhere in your template, but make sure it is outside the comments form. A good place would be right after the ending </form> tag for the comments form: <?php show_manual_subscription_form(); ?>