It is always good to avoid as many plugins as you can and instead implement the feature with a code, since too many plugins hamper the loading time of the website and thus leading to a very slow website.

How to implement Related Posts in Wordpress without any plugins?

It is very simple. Just go to your single.php file in Wordpress dashboard. (Dashboard -> Appearance -> Editor -> Single.php) and put this code where you want to display the posts.

<?php
$tags = wp_get_post_tags($post->ID);
if ($tags) {
$tag_ids = array();
foreach($tags as $individual_tag) $tag_ids[] = $individual_tag->term_id;

$args=array(
'tag__in' => $tag_ids,
'post__not_in' => array($post->ID),
'showposts'=>5, // Number of related posts that will be shown.
'caller_get_posts'=>1
);
$my_query = new wp_query($args);
if( $my_query->have_posts() ) {
echo '<h3>Related Posts</h3><ul>';
while ($my_query->have_posts()) {
$my_query->the_post();
?>
<li><a href="<?php the_permalink() ?>" rel="bookmark"
title="Permanent Link to <?php the_title_attribute(); ?>">
<?php the_title(); ?></a></li>
<?php
}
echo '</ul>';
}
}
?>

That is all. Now it will automatically generate all the related posts based on the tags used in the particular post. Do you also have any such methods? Then do let me know of it via comments.