Step 1:
From the store homepage where you placed your order, use the "Sign in" option instead of the store code box. This is the email and password you entered as part of placing your order initially.
(If you have forgotten this you can use the "Forgot Password" option in the popup that follows)
If you do not see this option below the store code box, you are already logged in. You can tell this if you see a circle with your initial in the top-right corner of the page. Click this and then "Orders" to get to the next step.
Step 2:
From the menu on the left, select the "Olive Payment" section.
In the middle section of this page you should see an option to make a payment, with a dropdown to select a pre-selected amount, or at the bottom of this list, the option to pay the full outstanding amount.