Binding is done with Game Center. If you're already logged into Game Center, your character is automatically bound. You can bind multiple characters on multiple devices to the same Game Center account.
To retrieve a character, go to "Character Transfer" and select the character you wish to retrieve.
If you've previously bound a character using the old email binding system, you can retrieve it with your email address and the code we sent to you (also in "Character Transfer"). It will then be bound to your Game Center account.