How to change the menu mark up with walker_nav_menu class?

I just posted an answer to this here: How to create this custom menu walker?

Basically, you want your start_el and end_el to looks something like this:

function start_el(&$output, $item, $depth=0, $args=array()) {
   $output .= '<a href="#"><span>' . esc_attr($item->label);
}

function end_el(&$output, $item, $depth=0, $args=array()) {
    $output .= '</span></a>';
}