WordPress.org

1. Upload this directory to the 'wp-content/plugins/' directory
2. Add the read and write permissions of 'wp-content/uploads' directory to the user of the web server
3. Activate the plugin through the 'Plugins' menu in WordPress
4. Open extenal CSS file editor through 'Appearance' -> 'External CSS' menu
5. Edit the CSS file

Updating

External CSS version 0.2.0 and older versions have a bug that removes CSS file when updating the plugin. To solve the problem, copy or rename wp-content/plugins/external-css/css/custom.css to wp-content/uploads/external-css/custom.css before trying to update the plugin.