What is the correct way to use rel=”nofollow”?

On wp-cms I have lines in contact.php:

< ?php if($facebook_url) { ?><li class="foot-facebook"><a href="<?php echo facebook_url; ?> ">Facebook</a></li><?php } ?>

In CSS (style.css):

.foot-twitter a { background:url(images/twitter.png) no-repeat; }
.foot-facebook a { background:url(images/facebook.png) no-repeat; }

Question – how to close a link set through the theme settings from indexing, Where and what to insert?

Answer 1, authority 100%

<a href="..." rel="noindex,nofollow">some text</a>