Once youve created the refund item button as per the tutorial to refund products you simply add them to the ticket as if you were selling them, then select the item and press the refund button, then go to settle screen to process refund.
This is in its very basic form, more complicated would have refund accounting, change of ticket type to refund, possible a separate refund receipt to print as i do so you can het a customer signature etc
A bit more complex is using the open closed ticket actions and cancel payments actions to reopen the original ticket, cancel the payments and calculations (charges) and refund the items
There is a couple of ways some more simple than others, have a search for refunds in the forum there are a couple of tutorials about it that might help you