Download the plugin from github, https://github.com/Giuseppe-Mazzapica/CleverRules, after you have unzipped the file, you can rename the folder `CleverRules-master` to `CleverRules`. After that, put in plugins folder and activate.

To make use of the plugin, you have to write a configuration plugins, because the routing rules cannot be wrote on theme `function.php`.

Your plugin will be something like:

<?php
/**
 * Plugin Name: My Rules
 * Author: Me
 */

add_action('plugins_loaded', 'register_my_rules');

function register_my_rules() {

  if ( ! function_exists('register_clever_rule') ) return;

  // RULES

  register_clever_rule(/items/%s/%d')
    ->query( 'pagename=items&string=[0]&number=[1]' )
    ->vars( array('string','number') ); // make wordpress recognize this additional query variables

}

Save this plugins in plugins folder and activate it too. See plugin doc page http://rules.zoomlab.it/documentation/ for more info on plugin use.