https://wpcrumbs.com/how-to-display-related-posts-without-a-plugin/
https://netmospherics.com/blog/wordpress-related-posts-without-plugin/

code นี้ไม่อัพเดท
http://www.wpbeginner.com/wp-themes/how-to-add-related-posts-with-a-thumbnail-without-using-plugins/