Visit the Delta Airlines website. If you have a Delta SkyMiles account, log in to your account using your SkyMiles number, SkyMiles PIN and password.
Click the "Book a Flight" tab.
Click the "View or Redeem Certificates & eCredits" link that is located at the bottom of the booking window.
Enter the 13-digit eCredit confirmation number. To use multiple eCredits, click the "View or Redeem Multiple Certificates & eCredits." You can use up to three confirmation numbers per individual booking.
Click "Continue."
Search for available flights. When you see the flight you would like take, select the option to book the flight. If the eCredit is insufficient to cover the cost of the flight, enter valid credit card information to pay the difference.
If the flight costs less than your eCredit, you will receive a new eCredit number for the difference. The new eCredit will bear the same expiration date as the original voucher.