creating a custom walker class

I am currently developing a theme for wordpress, and would like to add some data values to the list items that are returned.

Here is the basic structure of my nav:

<div id="nav" class="grid_9 omega">
                <ul class="navigation">
                    <li data-slide="1">Top Slide</li>
                    <li data-slide="2">Parallax Scrolling</li>
                    <li data-slide="3">Grid</li>
                    <li data-slide="4">Credits</li>
                </ul>
            </div>

I have created a walker class which is here (although not 100% how i want it):

class description_walker extends Walker_Nav_Menu
{

  function start_el(&$output, $item, $depth, $args)


{


global $wp_query;
       $indent = ( $depth ) ? str_repeat( "t", $depth ) : '';
           $class_names = $value = '';
       $classes = empty( $item->classes ) ? array() : (array) $item->classes;

       $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
       $class_names = ' class="'. esc_attr( $class_names ) . '"';

       $output .= $indent . '<li data-slide="'. $item->ID . '" id="menu-item-'. $item->ID . '"' . $value .'>';

       $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
       $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
       $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
       $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

      // $prepend = '<strong>';
      // $append = '</strong>';
       $description  = ! empty( $item->description ) ? '<span>'.esc_attr( $item->description ).'</span>' : '';

       if($depth != 0)
       {
                 $description = $append = $prepend = "";
       }

        $item_output = $args->before;
        $item_output .= '<a'. $attributes .'>';
        $item_output .= $args->link_before .$prepend.apply_filters( 'the_title', $item->title, $item->ID ).$append;
        $item_output .= $description.$args->link_after;
        $item_output .= '</a>';
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
        }

}

I am then calling wp_nav_menu like this:

<?php wp_nav_menu( array(
 'container' => 'div',
 'container_id'    => 'nav',
 'container_class' => 'grid_9 omega',
 'echo' => true,
 'before' => '',
 'after' => '',
 'link_before' => '',
 'link_after' => '',
 'depth' => 0,
 'walker' => new Walker_Nav_Menu())
 );?>

The menu is not outputting correctly, but that is purely because i dont fully understand how the walker class. Can anyone she some light?