How to use $query->set in a custom .php page?

this is my custom.php file.

require_once("wp-load.php");
global $wpdb;

$query->set('posts_per_page', 50 );           
$query->set('orderby', 'meta_value_num');
$query->set('meta_key', '_thumbs_rating_up');
$query->set('date_query', array( 'after' => '1 month ago' ));   
$query->set('order', 'DESC');   

How can a start this query now?