Struggling to get menu to work with Walker Class

Hello

Me again - sorry! :)

I'm trying to implement the Walker Class in my site, and have tried various external tutorials - but just can't get it working.

This is what I'm doing:

Downloaded file, placed it in theme directory.

I created a new menu called "Primary" - and I set it to be the Header Menu. I added all my pages to the menu.

In functions.php I am pasting the following code at the END:

// Register Custom Navigation Walker require_once('wp_bootstrap_navwalker.php');

In header.php my code is currently as follows (no changes yet):

<div class="navbar navbar-inverse navbar-fixed-top" role="navigation"> <div class="container"> <div class="navbar-header"> <button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse"> <span class="sr-only">Toggle navigation</span> <span class="icon-bar"></span> <span class="icon-bar"></span> <span class="icon-bar"></span> </button> <a class="navbar-brand" href="<?php bloginfo( 'url' ); ?>"><?php bloginfo( 'name' ); ?></a> </div>

    <div class="navbar-collapse collapse">

      <?php 
        $args = array(
          'menu'        => 'header-menu',
          'menu_class'  => 'nav navbar-nav',
          'container'   => 'false'
        );
        wp_nav_menu( $args );
      ?>

    </div>

AFTER:

<?php $args = array( 'menu' => 'header-menu', 'menu_class' => 'nav navbar-nav', 'container' => 'false', 'fallback_cb' => 'wp_bootstrap_navwalker::fallback', 'walker' => new wp_bootstrap_navwalker()) ); wp_nav_menu( $args ); ?>

Would appreciate any help - thank you! :D