On a Customer Account I have an Invoice and a Credit Note. How do I match them off against each other?

Thanks for visiting the QuickBooks Community, @john.berry. 

Allow me to help you apply a credit note to your customer’s invoice in QuickBooks Online.

Here's how to manually apply a credit note to an invoice. This process gives you control over which open invoice you can apply a credit note to.

1. Click the + New icon and select Receive payment or Receive invoice payment.
2. Enter the customer’s name, then fill out the required fields.
3. In the Credits section, select the credit note. Then, in the Payments column, enter how much credit your customer wants to use.
4. Make sure the total to pay (if any) is correct after applying the credit note.
5. Select Save and close.

The steps above will help you apply the credit to your customer’s open invoices. For your reference, you may check out this article: Apply a credit note or create and apply a delayed credit.

Please let me know if you have more questions. I’ll be here to help. Wishing you the best!