Following your agreement on a product return, you should receive a package from the customer. Once you have received that package, you must create a credit slip from the order's page. It can also be called "credit note" or "credit memo".
Credit slips are not created in the "Credit Slips" page, but only listed there. This page is also used to generate a PDF of your credit slips.
Delivery Slips Options
There is only one option on the page:
...
Do not forget to save your changes.
Creating a Credit Slip
In your list of orders ("Orders" page), click on the order for which the customer is returning some items for more details – this will only work if have already indicated that the customer can send products back.
...