making my own “related pages” / “pages you might like” section

Question

Function Reference/get the tags « WordPress Codex

Class Reference/WP Query « WordPress Codex#Tag_Parameters

add some code like this to content-single.php of your child theme of Twenty Eleven:

$tags = array();
$posttags = get_the_tags();
if( $posttags ) :
  foreach( $posttags as $tag ) { $tags[] = $tag->term_id; }

  $related_query = new WP_Query( array('tag__in' => $tags));
  if( $related_query->have_posts() ) : 
    while( $related_query->have_posts() ) : 
    $related_query->the_post();
      /*whatever you want to output; for instance:*/
      echo '<a href="'; the_permalink(); echo '">'; the_title(); echo '</a><br />';
    endwhile; 
  endif;
endif;