Hello medicalpractione,

Marking your invoices as paid is easy. Let me guide you with the steps.

1. Click Create (+).
2. Click Receive payment.
3. Click the customer name from the drop-down. You can also click the Find by invoice no. button.
4. Check the box next to the invoice number you want to pay. Make sure you enter the full invoice amount. 
5. Click Save and close.

Once you're done, go to the customer's profile and open the invoice to see the PAID status.

Here's how:

1. Click Sales. Then, Customers.
2. Click the customer's name. Then, click the invoice under Transaction List to open.
3. You'll see the payment status at the top of the invoice.
4. Click Save and close.

See attached screenshots below.

You can learn more about receiving payments in this article: Receive Payments in QuickBooks.

I've also got an article about income and expenses in case you need for your future tasks.

Don't hesitate to reach out if you have additional questions. We're always here to help.