how to filter by tag params?

blog.php

<?php snippet('header') ?>
<?php snippet('menu') ?>
<?php 
/* WHRE DO I PUT THE FOLLOWING CODE SO THAT /blog/tag:SomeTagName works?
   When I click on a tag link, it still displays all the articles even if the tags don't match.
 */ 
if(param('tag')) {

  $article = $pages->find('blog')
                    ->children()
                    ->visible()
                    ->filterBy('tags', param('tag'), ',')
                    ->flip()
                    ->paginate(10);

} else {

  $article = $pages->find('blog')
                    ->children()
                    ->visible()
                    ->flip()
                    ->paginate(10);

}

?>


<section class="content blog">
  
  <h1><?php echo html($page->title()) ?></h1>
  <?php echo kirbytext($page->text()) ?>
  
<!-- FOR EACH LOOP BEGIN -->
  <?php foreach($page->children()->visible()->flip() as $article): ?>
  <article>
    <h1><?php echo html($article->title()) ?></h1>
    <p><?php echo excerpt($article->text(), 300) ?></p>
    <a href="<?php echo $article->url() ?>">Read more…</a>
    <?php echo $article->tags() ?>
  </article>

  <?php endforeach ?>
<!-- FOR EACH LOOP END -->

<hr/>
<hr/>
<!-- CLICKABLE TAG CLOUD GENERATOR -->
<?php $tagcloud = tagcloud($pages->find('blog'), array('limit' => 20)) ?>
<ul>
<?php foreach($tagcloud as $tag): ?>
  <li><a href="<?php echo $tag->url() ?>"><?php echo $tag->name() ?></a></li>
<?php endforeach ?>
</ul> 



<hr/>
</section>

<?php snippet('footer') ?>