The system makes the process of creating customs, or pro forma, invoices easy - all you need do is untick the 'Docs only' tick box.
When you submit the consignment for creation the system will use the 'Val for Customs' and 'Description' and use them to create a customs invoice. If you need to add more detail you will need to use the 'Edit' consignment screen to add extra line items to the invoice.
Once created please print a copy for attachment to your shipment.