On the left-hand side of the screen in the navigation bar, select Dashboard.
If you have an outstanding balance, your account balance will be displayed at the top of the screen in a window with an exclamation mark that looks like this:

Select the Payment button to the right of this window. This will take you here:

*Alternatively, you can also access the Payment screen by selecting My Account on the left-hand side of the screen, then clicking the plus sign icon titled Payment in the bar under MY ACCOUNT
*You can also access the Payment screen by selecting My Account on the left-hand side of the screen, then clicking the plus sign icon titled Payment.
These options will bring you to a screen titled Add Payment that looks like this:

Enter your billing address and credit card details in this window.
Scroll down to the bottom of the screen to Open Invoices.
To make a payment on a specific invoice, use the selection window to the right of the invoice with a $ symbol. Type the amount you want to pay for that invoice in this window. It will look like this:

Once you have entered your billing address and credit card info, then selected the amount you want to pay on your invoice, a button will appear titled Submit Payment.
Click the Submit Payment button to make a payment.