How to use custom previous/next link ?

Do this to output only links for previous and next pages:

<?php previous_posts_link ( 'Previous' ) ?>
<?php next_posts_link ( 'Next' ); ?>

Then add filters to your functions.php to add a class to each link:

function next_posts_link_css ( $content ) {
    return 'class="next"';
}
add_filter( 'next_posts_link_attributes', 'next_posts_link_css' );

function previous_posts_link_css ( $content ) {
    return 'class="prev"';
}
add_filter( 'previous_posts_link_attributes', 'previous_posts_link_css' );

Then style the .next and .prev links using CSS.