Find the following bit of code in the “wp_shopping_cart.php” file:
if ($_POST)
once you find it you will need to add the following at the end of that if statement block:
$checkout_url = "www.your-checkout-url.com/checkout");
$redirection_parameter = 'Location: '.$checkout_url;
header($redirection_parameter);
Please note that a bit of PHP knowledge is required to be able to do this modification (I can’t really teach you PHP).