If you want to use related posts by taxonomy plugin to display list of related posts automatically after your WordPress posts, you will need to add a code snippet in your (child) theme’s functions.php file.
Here is an example with four related posts after the content on all single post pages.
Note: See below this code snippet how you can change it to suit your needs.
add_filter( 'the_content', 'add_related_posts_after_post_content' );
function add_related_posts_after_post_content( $content ) {
//check if it's a single post page.
if ( is_single() ) {
// check if we're inside the main loop
if ( in_the_loop() && is_main_query() ) {
// add your own attributes here (between the brackets [ ... ])
$shortcode = '[related_posts_by_tax posts_per_page="4"]';
// add the shortcode after the content
$content = $content . $shortcode;
}
}
return $content;
}
Source : The plugin’s doc