Code to include related posts to function.php, so that related posts are shown at the end of each post http://www.wppicker.com/how-to-add-responsive-related-posts-without-plugin/

include-related-posts-function.php

/*Include the code to generate related posts*/
include_once 'functions/custom-related-posts.php';

/*include related posts to the end of a post*/
function add_related_post_after_post_content($content) {
	if(!is_feed() && !is_home() && is_singular() && is_main_query()) {
		
		$content=wpp_related_posts($content);
	}
	return $content;
}
add_filter('the_content', 'add_related_post_after_post_content');