Dropdown filter by custom field

I have a problem: I need to add a filter associated to a Custom Field.

I installed the plugin “Advanced Custom Fields” and I created a field type “date picker” with key “Fecha-de-Estreno”, until here everything ok.

Now I want to add in my theme a new filter associated with this new field and do not know how I can do.

I have a template called “dropdown-filter.php” with this code:

<?php
global $gp_settings;

// Dropdown Filter URLs
$page_url = $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"];
$short_page_url = $_SERVER["REQUEST_URI"];
$cat_id = get_query_var('cat');
$category_url = get_category_link($cat_id);

if( is_search() ) {

  $category_url = $category_url.'?s='.esc_html($s).'&amp;'; 

} else {

  $term = get_term_by('slug', get_query_var('term'), get_query_var('taxonomy'));
  $tax_name = get_taxonomy(get_query_var('taxonomy'));

  if( $tax_name && ! $tax_name->hierarchical ) {

    if( $tax_name->name == 'release_date' ) {
      $cat_slug = $theme_review_tag_1_slug;
    } elseif( $tax_name->name == 'genre' ) {
      $cat_slug = $theme_review_tag_2_slug;         
    } elseif( $tax_name->name == 'rating' ) {
      $cat_slug = $theme_review_tag_3_slug;
    } elseif( $tax_name->name == 'director' ) {
      $cat_slug = $theme_review_tag_4_slug;
    } elseif( $tax_name->name == 'producer' ) {
      $cat_slug = $theme_review_tag_5_slug;
    } elseif( $tax_name->name == 'screenwriter' ) {
      $cat_slug = $theme_review_tag_6_slug;
    } elseif( $tax_name->name == 'studio' ) {
      $cat_slug = $theme_review_tag_7_slug;
    } elseif( $tax_name->name == 'starring' ) {
      $cat_slug = $theme_review_tag_8_slug;
    } else {
      $cat_slug = $tax_name->name;
    }

  } else {
    if( get_option("permalink_structure") ) {
      $cat_slug = $theme_review_cat_slug;
    } else {
      $cat_slug = "review_categories";
    }
  }

 if( get_option("permalink_structure") ) {
    $category_url = home_url() . '/'. $cat_slug . '/' . $term->slug . '/?';
  } else { 
    $category_url = home_url() . '?' . $cat_slug . '=' . $term->slug . '&amp;'; 
  }

}

?>

<div id="dropdown-filter">
  <div class="order-by-text"><?php _e('Sory By', 'gp_lang'); ?>:</div>
  <form class="order-by-form">
    <select>
      <option value="<?php echo $category_url; ?>orderby=date"<?php if(preg_match('/orderby=date/', $page_url)) { ?> selected<?php } ?>><?php _e('Date', 'gp_lang'); ?></option>        
      <option value="<?php echo $category_url; ?>orderby=title"<?php if(preg_match('/orderby=title/', $page_url)) { ?> selected<?php } ?>><?php _e('Title', 'gp_lang'); ?></option>                             
      <?php if($gp_settings['orderby_site_rating'] == true) { ?><option value="<?php echo $category_url; ?>gdsr_sort=review&gdsr_multi=<?php echo $theme_site_multi_rating_id; ?>"<?php if(preg_match('/gdsr_sort=review/', $page_url)) { ?> selected<?php } ?>><?php _e('Site Rating', 'gp_lang'); ?></option><?php } ?>   
      <?php if($gp_settings['orderby_user_rating'] == true) { ?><option value="<?php echo $category_url; ?>gdsr_sort=rating&gdsr_multi=<?php echo $theme_user_multi_rating_id; ?>"<?php if(preg_match('/gdsr_sort=rating/', $page_url)) { ?> selected<?php } ?>><?php _e('User Rating', 'gp_lang'); ?></option><?php } ?>                               
    </select>
  </form>
  <div class="order-text"><?php _e('Order', 'gp_lang'); ?>:</div>
  <form class="order-form">
    <select>
      <option value="<?php if(preg_match('/orderby=/', $page_url) OR preg_match('/gdsr_sort=/', $page_url) OR !get_option("permalink_structure")) { ?><?php echo $short_page_url; ?>&order=desc&gdsr_order=desc<?php } else { ?><?php echo $short_page_url; ?>?order=desc<?php } ?>"<?php if(preg_match('/order=desc/', $page_url)) { ?> selected<?php } ?>><?php _e('Descending', 'gp_lang'); ?></option>                          
      <option value="<?php if(preg_match('/orderby=/', $page_url) OR preg_match('/gdsr_sort=/', $page_url) OR !get_option("permalink_structure")) { ?><?php echo $short_page_url; ?>&order=asc&gdsr_order=asc<?php } else { ?><?php echo $short_page_url; ?>?order=asc<?php } ?>" <?php if(preg_match('/order=asc/', $page_url)) { ?> selected<?php } ?>><?php _e('Ascending', 'gp_lang'); ?></option>              
    </select>
  </form>
</div>

The plugin (Advanced Custom Fields) support tells me I have to add this but not that I have to do.