Add additional menu

I ALWAYS struggle with WP Menu’s, just can’t get my head around them….

I am working on a theme that only supports one menu so I need to add an additional menu.

The menu code is:-

<?php $args = array('container' => 'nav','container_class' => 'sidemenu','walker'=>new ows_walker_nav_menu); ?>
<?php wp_nav_menu($args); ?>

Function code:-

/* ========================================================================================================================

	Register Navigation

	======================================================================================================================== */
register_nav_menu('main', 'Main navigation menu');
class ows_walker_nav_menu extends Walker_Nav_Menu {

	// add classes to ul sub-menus
	function start_lvl( &$output, $depth ) {
	    // depth dependent classes
	    $indent = ( $depth > 0  ? str_repeat( "\t", $depth ) : '' ); // code indent
	    $display_depth = ( $depth + 1); // because it counts the first submenu as 0
	    $classes = array(
	        'sub-menu',
	        ( $display_depth % 2  ? 'menu-odd' : 'menu-even' ),
	        ( $display_depth >=2 ? 'sub-sub-menu' : '' ),
	        'menu-depth-' . $display_depth
	        );
	    $class_names = implode( ' ', $classes );

	    // build html
	    if ($display_depth >= 1) {
	    	$output .= '<nav class="sub-menu-container">' . "\n";
	    }
	    $output .= "\n" . $indent . '<ul class="' . $class_names . '">' . "\n";
	}

	function end_lvl( &$output, $depth ) {
    // depth dependent classes
    $indent = ( $depth > 0  ? str_repeat( "\t", $depth ) : '' );
    $display_depth = ( $depth + 1); // because it counts the first submenu as 0
    $output .= "\n" . $indent . '</ul>' . "\n";
    if ($display_depth >= 1) {
    	$output .= '</nav>' . "\n";
    }
  }
}

I have tried to change this a few times but the theme either breaks or the change doesn’t work. Could someone help me out and point me in the right direction please