Register an Amex Prepaid REDcard

First, you need to find a REDcard. At the time of this writing, it’s only available the following states: Alabama, Arizona, California, Colorado, Indiana, Maryland, Massachusetts, Michigan, Mississippi, New Mexico, North Carolina, Ohio, Oklahoma, Pennsylvania, South Carolina, Tennessee, Virginia, Wisconsin. You can get an up to date list in the FAQs here.
Once you have the card, go to Amex’s REDcard homepage and select “Register” in the upper right hand corner.
Enter the card number, security code, and your birthday. If this card was bought under someone else’s information, leave the “Date of Birth” box empty.
Enter the rest of your information on the next page, and submit the application. The information you use to register for the card, does not have to be the same as the information used when the card was purchased.

Notes

It will take 5-7 business days to receive your new REDcard.
You can only register for an account, if you do not have an open Bluebird or Serve. You can only have one of the three.
There is not a waiting period between canceling your Bluebird or Serve account and opening a REDcard.
This card is different from the prepaid Amex for Target card.
If you run into issues registering your card, check out this post.