Once you've received your trade-in offer and submitted your order, you will receive an email with the shipping label for your trade-in. If you do not receive this shipping label within a couple of hours after your order, be sure to check your spam folder.
Make sure that you have disabled the activation lock on your phone, backed up and removed your personal data (including your memory card or SIM card, if your phone has one) and that you factory reset it. Then, place the phone in a box with enough padding to keep it safe in transit.
Your shipping label will actually contain a receipt to place inside the box, and two labels for the outside: a Merchandise Return Label, and a Lithium Battery label. Be sure to seal the box, and then use standard shipping via USPS.
If you are trading in multiple phones, be sure to pack and ship each phone separately.