How to create shortcode of this PHP code

This should do it..

   function future_posts_function($atts){
    extract(shortcode_atts(array(
        'poststatus' => 'future',
        'order'         => 'DESC',
        'showposts' => 10,
    ), $atts));

    $return_string = '<ul>';
    query_posts(array('post_status' => $poststatus, 'order' => $order, 'showposts' => $showposts));
    if (have_posts()) :
        while (have_posts()) : the_post();
            $return_string .= '<li>'.get_the_title().'</li>';
        endwhile;
    endif;
    $return_string .= '</ul>';

    wp_reset_query();
    return $return_string;
}
add_shortcode('future_posts', 'future_posts_function');