PayPal IPN

[newbrier]

I have installed osCommerce and everything works fine. I'm trying to setup PayPal to handle my order processing. I'm familar with paypal's ipn from work with Commerce.cgi. I've downloaded and completed the customization areas of PayPalIPN.php but there is no direction on where to put the files. I put them in /includes/modules/payment/ but then modules.php errors out. I've searched the forums and most of the links on this topic no longer work. Please point me in the right direction if there are more detailed instructions on how to set this up. Thanks.