Yunus emre ye teşekkür edebilirsin ustad..
bu php kodlarını single.php ye ekle , css kodlarını da style.css ye , canavar gibi çıkacak

<!-- benzer yazılar son -->

<div class="baslikblok">Benzer Yazılar</div>
 <ul class="benzerler">
 <?php
$categories = get_the_category($post->ID);
if ($categories) {
   $category_ids = array();
   foreach($categories as $individual_category) $category_ids[] = $individual_category->term_id;
 
   $args=array(
      'category__in' => $category_ids,
      'post__not_in' => array($post->ID),
      'showposts'=>4, // Gösterilecek benzer yazi sayisi
      'caller_get_posts'=>1
   );
 
        $my_query = new wp_query($args);
   if( $my_query->have_posts() ) {
      while ($my_query->have_posts()) {
         $my_query->the_post();
      ?>
      
        
         <li>
<?php if (function_exists('has_post_thumbnail')) {
 if ( has_post_thumbnail() ) { ?>
    <div class="thumb4">
    <a href="<?php the_permalink(); ?>"><?php the_post_thumbnail(); ?></a>
       <p><?php kisa_baslik(); ?></p>
    </div>

   <?php } } ?>

         </li>
         
      <?php
      }
      echo '<div class="temizle"></div>';
   }
wp_reset_query();
}
?> 

<!-- benzer yazılar son -->

css

ul.benzerler{ float:left; margin:10px 0px 0px 10px; padding:0px;}
ul.benzerler li{ list-style:none; float:left; padding:0px 5px 10px 5px; }
.baslikblok{ font-weight:bold; min-height:30px; line-height:30px; font-size: 11px; margin: 0px 13px 0px 15px; padding-left:20px; color: #7e7e7e; text-decoration: none; text-shadow: 0 1px #fff; background: #f3f3f3; -webkit-box-shadow: inset 0px 0px 2px #fff; -moz-box-shadow: inset 0px 0px 2px #fff; box-shadow: inset 0px 0px 2px #fff; border: 1px solid #DFDFDF; border-radius: 5px; -moz-border-radius: 5px; -webkit-border-radius: 5px;}