WordPress.org

1. Upload wp-css folder to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Set the permissions for /wp-css/ folder to 777
4. Go to Settings > WP CSS in the admin site. This setup the plugin.
5. Your default stylesheet (style.css) is automatically scanned
6. Use <?php wp_css('path/to/css/file.css'); ?> to link to other css files

i.e. <link rel="stylesheet" href="<?php wp_css('path/to/css/file.css'); ?>" type="text/css" media="screen" />