In order for a customer return to be issued, the status of the sales order has to be invoiced. If the status is 'in process', there won't be a customer return button. Once you generate an invoice, a customer return button will appear.
When you clcik on it, you will enable the quantity field.
You can leave the entire quantity or type in the partial quantity. If the customer didn't return an item that is on the list, just type in a zero in the quantity field. Click on 'save' and post the document.
You will get a tag that says 'returned'.
This document will be right under the original sales order on the sales order list.