Login to the PSN website, click on your profile at the top right of the screen, and select the "Redeem Code" menu;
Enter the PSN Code that you have received from the transaction and then click "Next";
You will see the confirmation windows giving you the details of your current balance and the amount of the redeemed voucher. Please make sure the amount is right and then click "Add Funds";
Done! Your PSN Wallet will be automatically increased once the redemption process is complete.