Do you mean related articles underneath your main Facebook Instant Article on Facebook?
You edit this code block – <footer> in your html edit on Facebook –
<footer>
<ul class=”op-related-articles”>
<!– Credits for your article –>
<aside>The footer section can be used for acknowledgements, author bios, related articles, or any other supplemental information.</aside>
<!– Copyright details for your article –>
<small>? Facebook</small>
</footer>
All I do is copy paste this bit….wrapped in <footer></footer> AND edit those urls to your own site article urls – you can add another line too to make three related – just copy paste a
- etc
<ul class=”op-related-articles”>
Works really well