You will have received a letter explaining you are eligible for a pre-selected gift card. The voucher will be sent with the gift card.
The pre-selected gift card has already been processed on your behalf. Therefore, you simply need to take it to the chosen retailer and pass it across to the cashier at the till when paying for your shopping. After paying for your shopping, your receipt will give you an up to date balance on your gift card.
The value of your shopping will be deducted from the balance of the gift card. You can use your gift card multiple times, or until the balance is £0.