As soon as an invoice is generated, which is 14 days before the due date, you will be able to see it within your portal. You can do this by logging into your portal and selecting either "Invoices" at the top right or "View All" in the invoices box.
Once you have done so you will see a history of your invoices. Select the unpaid invoice if you wish to pay the invoice early, or select one of the other invoices if you wish to view past invoices.
This will give a full invoice and if you wish to pay the invoice early you can with the "Pay Now" option. Otherwise the card on record will automatically be billed on the due date, or the direct debit taken if this has been set up.