How/where to add additional stylesheet to WP

The WordPress API provides the functions wp_enqueue_style() and wp_enqueue_script() for including stylesheets and JavaScript.

These functions should be used instead of hardcoding style or script tags within HTML.

// Runs wpse_enqueue_styles() on the wp_enqueue_scripts() hook.
// (Yes, we're enqueuing a stylesheet here, but the proper hook is wp_enqueue_scripts.
add_action( 'wp_enqueue_scripts', 'wpse_enqueue_styles' );
function wpse_enqueue_styles() {

        wp_enqueue_style(
            // Handle
            'example-font',

            // Source (URL) for stylesheet.
            // Here, the stylesheet resides at /theme-name/css/MyFontsWebfontsKit.css
            get_stylesheet_directory_uri() . '/css/MyFontsWebfontsKit.css',

            // Dependencies for stylesheet. None in this case.
            array()
        );
}

Place the code above in your theme’s functions.php file. Note that the “wpse_” prefix should be changed to match whatever is used by your theme.