1. Open a new credit memo. Click + New, then Credit memo under the Customers column.
2. Select the customer and add the credit lines. Choose the customer, then add the same product or service that was originally billed (or whatever the credit actually applies to) with the amount you're crediting them.
3. Save it. The credit memo now shows as an available credit on that customer's account.
4. Apply it to an open invoice if one exists. Open the invoice the credit relates to, or use Receive payment for that customer; the available credit memo shows up there and you can apply some or all of it against the balance due.
5. If there's no open invoice yet, the credit sits available and applies automatically the next time you invoice that customer, depending on your settings.
6. To refund the customer directly instead of leaving it as credit, QuickBooks gives you the option right on the credit memo screen to issue the refund from a bank account or through QuickBooks Payments, rather than waiting for the next invoice to absorb it.