An 'Apply voucher' link will be visible on your bill. When selected, you will be presented with any vouchers that have been added to your Wallet for the restaurant you're dining in. You will also be able to type a voucher code if the offer is not available in your wallet. When adding an offer code, you will be asked if you wish to redeem it. Once you've done this you will see the offer uploading in the background. As soon as this is complete it will confirm the voucher has been applied, and the new discounted balance will show on your bill.
Please note, you have to be logged in to be able to apply vouchers from your wallet. If you are not logged in, you will have to manually enter a voucher code.