How to show custom error messages below text field?
You need go to your theme's template folder and edit page.tpl.php'

Look for &lt;?php print $messages; ?&gt; and move it below &lt;?php print render($page['content']); ?&gt;

Thanks for the note @Kevin. If you wish, you can add if statements around the &lt;?php print $messages; ?&gt; that check the page url and/or content type if you need to have them appear below only for certain pages.