You would need to output them into the links, for example;
<?php
// whatever function would be used to get the link for the current page
$link = get_the_permalink();
?>
<a href="<?php echo $link; ?>?myvar=ASC">123</a>
<a href="<?php echo $link; ?>?myvar=DESC">321</a>
then you need to alter the query run on the page to add a meta_query. You can find an example of that in the link I gave before and there is more information in the WP documentation http://codex.wordpress.org/Class_Reference/WP_Query