ASN and EDI file assistance for sending file

We are looking to ntegrate our warehouse system with Sainsbury's system when items are dispatched. We ave the file created for pallets and for delivery but need some help in how to send the file over to the Sainsbury's system. Looking at the options we are looking to send an edi asn using GXS ftp. But finding out how to do this is proving difficult so looking for someone with knowledge of how this is done and has done it before. Our system uses PHP to create the file, so we would want this to send through the GXS system.