How do i set up a promissory note with a customer. I.e. the customer is making payments on a debt that he owes us.

Hi there, @cdougherty.

The option to set up promissory notes is unavailable in QuickBooks Online (QBO).

I'd suggest looking for a third party that would help you accomplish your goal in QuickBooks. Simply sign in to your QBO account and go to the Apps menu. Then, search for the app from the Search field. 

Moreover, you can set up a recurring transaction for a customer's payment. That way, it would be easy for you to track their payments until they settle the debt. Let me show you how. 

First, create a new template from scratch:

1. Go to Settings ⚙.
2. Under Lists, select Recurring transactions.
3. Click New.
4. Choose the type of transaction to create, and select OK.
5. Enter a Template name.
6. Choose a Type: Scheduled, Unscheduled, or Reminder.
7. Complete the remaining fields and click Save template.

In case you need to edit a recurring template, check out this article for further details: How to change a recurring transaction template in QuickBooks Online.
 

You're welcome to visit this thread again if you have other questions regarding QuickBooks Online. I'm always here to assist you.

@cdougherty   You can find a LOT of promissory notes on Google.  Search for "Promissory note pdf".