wp_enqueue_style( 'my-style', get_template_directory_uri() . '/css/my-style.css', false, '1.0', 'all' ); // Inside a parent theme
wp_enqueue_style( 'my-style', get_stylesheet_directory_uri() . '/css/my-style.css', false, '1.0', 'all' ); // Inside a child theme
wp_enqueue_style( 'my-style', plugins_url( '/css/my-style.css', __FILE__ ), false, '1.0', 'all' ); // Inside a plugin