You can get to the additional payment page in several different ways.
The first way is to go to the "Finance Tab> Payments."

Once on the payment page, click the "Add Payment" button.

Another way is to go to the "Finance Tab>Invoice page."

Locate the invoice and click the "$" button.

You can also access the additional payment page via the "Customers" tab.

Find and click on the customer you want to add payment to.

At the top, you will see a "+ Add Payment" button.

Once on the Add a Payment page, select the customer you're entering the payment.

If the customer has an unpaid invoice, it will show in the "Apply To" drop-down.

If they don't, you will only see "Add as a Customer Credit."

Choose where you want this payment to go.

Enter the payment amount or check the box if the payment is in full.

Choose the payment method. 

Note: Just choosing "Credit Card" in this drop does not charge the card.

When all fields are selected and filled out, click the "Add Payment" button.

Because the payment amount could be higher than the invoice you selected, and the additional amount would go to the customer's credits, a pop-up will appear and ask if you want customer credits to be automatically applied to any other outstanding or future invoices.

You will then be taken to the payment receipt, where you can download/print a pdf or email the receipt.