WordPress.org

1. Unzip "Improved Gallery" archive and put the folder into your "plugins" folder (/wp-content/plugins/).
2. Activate the plugin from the Plugins admin menu.

Advanced Usage

If you want to make it even easier to edit your gallery style, you can do this:

1. Copy the styles from 'gallery-style.css' into the main 'style.css' file in your theme (wp-content/themes/your-theme).

2. Edit improved-gallery.php and delete or comment out this line, like so:

   // add_action('template_redirect', 'improved_gallery_stylesheet');