Please follow the steps below:
Sign into your PSN Account. If you don’t have one, create one at playstation.com
Go to the PlayStation®Store on your console or via our website.
Select the “Redeem Codes” option (for PS Vita, first tap the Options icon at the foot of the screen).
Enter the 12 digit voucher code and then select “Continue”.
Follow the on-screen instructions to redeem your voucher code.