I need to make sure that QB applies a payment to oldest invoices first.

I'm here to guide you on how to apply a payment to the previous invoices, John Drake.

When you received a payment, make sure to select the right customer and toggle the dates you created the old invoices. You can do it manually by following the steps below:

1. Click the +New button and select Received Payment under Customers.
2. Select the name of the customer and filter the correct dates so QuickBooks will automatically generate all the invoices made on that date.
3. Review the transaction if this is the transaction you want the payment to be applied. Then, enter the amount.
4. Once everything is set, you can now click the Save and Close button.

Then, you can go to the Customers section to check and verify if the payment was applied to the old invoices of your customer. Go to Sales and select the Customers tab. Select the name of the customer to see all the transactions you've made for them. Same when you received the payment, you can filter it to a particular transaction as well as the date.

Also, you can run the Invoices Lists by Date reports in your book to see all the transactions. Also, you can customize it to make the report more personalize. 

Please get back here if you have other questions. I'd love to help you more! Take care!