The code is to go within the loop (so allows PHP), specifically this part which is right underneath the php call for the_content();
<?php if ( in_category ( 'comic' ) ) : ?>
<div class='comicbuttons'>
<span class='cbtn first'>FIRST</span>
<span class='cbtn back'>BACK</span>
<span class='cbtn next'>NEXT</span>
<span class='cbtn latest'>LATEST</span>
</div>
<?php endif; ?>
It can be seen in action here: https://www.flipsum.co.uk/410 (apologies for the mess, I’m renovating at the moment).
I’m no expert on php – I’ve managed to put together most of my own theme with a combination of guesswork and searching the forums, but the category links have stumped me completely. Any help would be greatly appreciated.