Add this into your single post template, and call related posts based on category.
<?php
$related = get_posts( array( 'category__in' => wp_get_post_categories($post->ID), 'numberposts' => 5, 'post__not_in' => array($post->ID) ) );
if( $related ) foreach( $related as $post ) {
setup_postdata($post);
?>
<ul>
<li>
<?php the_post_thumbnail( 'blog-feed-thumbnail' ); ?>
<a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a>
<a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title(); ?>">Read more ></a>
</li>
</ul>
<?php
}
wp_reset_postdata();
?>