If you have Quins Credit available this will show in your notification panel once you have logged into your ticketing account.
Simply add all tickets to your basket and proceed to checkout.
Once you get to the checkout screen you can choose your method of payment, at the top of the screen you will see any available Quins Credit and will need to select the balance you want to use towards your purchase(s). If there is any remaining balance you will need to pay for this via a Credit or Debit Card.