WordPress.org

1. Copy or upload the custom-pagination directory into your /wp-content/plugins/ directory
2. If you downloaded a zipped copy (i.e. custom-pagination.zip), use the 'Plugins' > 'Add New' section of WordPress to upload it
3. Activate the plugin through the 'Plugins' menu in WordPress

Done.

Actually, if you wish to use WordPress pagination when there is no Custom Pagination on a page, you may use the following PHP code:

// inside single.php
// displays the standard pagination when no custom pagination link is found
if ( ! cp_link_found() )
    wp_link_pages(array('next_or_number' => 'next', 'nextpagelink'=>'Next page »', 'previouspagelink'=>'« Previous page'));