Redeeming a free sandwich through the McDonald's survey is quick and simple. Start by visiting the official mcdvoice.com survey (https://mcdvoice.us.com/survey/) website and enter the 26-digit survey code printed on your recent McDonald's receipt. Answer a few questions about your dining experience honestly. After completing the survey, you will receive a validation code. Write this code on your receipt and take it to a participating McDonald's location. Present the receipt during your next visit to redeem your free sandwich, usually with the purchase of a drink, as per the offer terms.