1. On the united.com home page, enter the origin, destination and travel dates. Select the "Search" button.
2. Choose your flights and enter your contact information.
3. When you reach the Payment information page, select "United payment types." You will see an option to redeem a travel certificate.
4. Enter your certificate number or PIN and the issuer's last name and select "Apply."
Select the "Purchase" button to finish booking your flight.