Where to place code, and which bits to edit

alchymyth very kindly posted an example code that I might use to display lots of pages on a single page. The only problem is, I don’t know where to put it, or which bits to edit! I would warmly welcome any suggestions, and wouold like to express my gratitude in advance for your patience with me! The code is:

<?php
$args = array(
  'post_type' => 'page',
  'post__in' => array( 2, 5, 35, 67 ) //list of page_ids
);
$page_query = new WP_Query( $args );
if( $page_query->have_posts() ) :
echo '<div class="pages-on-page">';
//print any general title or any header here//
while( $page_query->have_posts() ) : $page_query->the_post();
echo '<div class="page-on-page" id="page_id-' . $post->ID . '">';
//print any output you want per page//
echo '</div>';
endwhile;
echo '</div>';
else:
//optional text here is no pages found//
endif;
wp_reset_postdata();
?>

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