Adding an external stylesheet to a plugin

I’m assuming your plugin is working, you just don’t get the stylesheet stuff to work?

In that case, you can use the wp_enqueue_scripts action.

http://codex.wordpress.org/Function_Reference/wp_enqueue_style#Using_a_Hook

For usage with a stylesheet in the pluginfolder, you can use it like this:

function my_plugin_name_scripts() {
    wp_enqueue_style( 'my-plugin-custom-styles', plugins_url('custom-styles.css' , __FILE__ ) );
}

add_action( 'wp_enqueue_scripts', 'my_plugin_name_scripts' );

Happy Coding,
Kuchenundkakao