When you are sent a store credit reward, you will automatically receive an email with the gift card that looks like this:
You can also always access your gift cards in the payouts tab of your affiliate interface by clicking the View gift card button for each payout.
When clicked, a pop-up displaying the gift card info will appear:
To redeem the gift card code, copy the code and paste it into the section labeled 'Gift Card' or 'Discount Code' (or similar) during the checkout process on the brand's store.