[Customize the_post_navigation] #WordPress

wp_custom_post_navigation.php

Use WordPress default function to get next and previous post title with permalink something like this:

    <?php  // FOR PREVIOUS POST
        $prev_post = get_previous_post(); 
        $id = $prev_post->ID ;
        $permalink = get_permalink( $id );
    ?>
    <?php // FOR NEXT POST
        $next_post = get_next_post();
        $nid = $next_post->ID ;
        $permalink = get_permalink($nid);
    ?>
    
Then just use this variable in your HTML structure like below:

<div class="next-timeline">
     <?php next_post_link( '%link', __( 'Next <span class="meta-nav">&rarr;</span>', 'twentyeleven' ) ); ?>
     <h2><a href="<?php echo $permalink; ?>"><?php echo $next_post->post_title; ?></a></h2>
</div>
<div class="previous-timeline">
     <?php previous_post_link( '%link', __( '<span class="meta-nav">&larr;</span> Previous', 'twentyeleven' ) ); ?> 
     <h2><a href="<?php echo $permalink; ?>"><?php echo $prev_post->post_title; ?></a></h2>                           
</div>

Hi, I'm a PHP newbie: can you please clarify for me where in the theme/WP structure I add this coding? Do I create a new PHP file? Thank you!