Click on the accounts icon (the person) in the top right menu.
Click on the vouchers tab on the left menu of the accounts dashboard.
In the redeem voucher section, enter the voucher code to be redeemed and click on check.
Confirm the amount for the voucher and then click on submit.
This has now been redeemed to you account. You can check out how to create vouchers here. You can see all of these steps in the video below: