How do I post a payment on account to a supplier account?

Hello, @David W.

You can create an expense transaction for this. It will associate your payment to your supplier. 

Let me guide you how:

1. On the left navigation pane, click on the +New icon
2. Under  Vendors column, select Expense.
3. On the Payee dropdown select your supplier/vendor.
4. Fill in the needed information then click Save and Close.

For your future reference, you also run a report with vendor totals: Run a report with vendor totals.

I'll be happy to assist you if you have additional questions, take care!