After you talk with them on the phone, they send you an e-mail describing the transaction.
Print that e-mail and at the bottom write: Trade-In Program.
Enclose the printed e-mail in a box with your racquets.
Ship them with whoever you want. (You can also use an old Tennis Warehouse UPS shipping return label as long as it is not too old.)
Once the racquets arrive to TW they will look them over.
Then, they will call you or send you an e-mail that has a password that you can use online to redeem for the amount that your racquets are worth.
Buy online what you want.