add custom field to custom post type

function products_custom_init() {

    $labels = array(
        'name' => _x('Products', 'post type general name'),
        'singular_name' => _x('Product', 'post type singular name'),
        'add_new' => _x('Add New', 'products'),
        'add_new_item' => __('Add New Product'),
        'edit_item' => __('Edit Product'),
        'new_item' => __('New Product'),
        'view_item' => __('View Product'),
        'search_items' => __('Search Products'),
        'not_found' =>  __('Nothing found'),
        'not_found_in_trash' => __('Nothing found in Trash'),
        'parent_item_colon' => ''
    );

    $args = array(
        'labels' => $labels,
        'public' => true,
        'publicly_queryable' => true,
        'show_ui' => true,
        'show_in_menu' => true,
        'show_in_nav_menus' => false,
        'query_var' => true,
        'rewrite' => array('slug','pages'),
        'capability_type' => 'post',
        'hierarchical' => true,
        'menu_position' => 5,
        'supports' => array('title','editor','thumbnail','excerpt',)
      );

    register_post_type( 'products' , $args );
}
add_action( 'init', 'products_custom_init' );