To start, you need to go to HTC's website, which you can access by clicking on the sourcelink, and signing up with your email address to pre-register for the deal. This must be done by April 4th and once you've pre-registered for the trade-in, you must purchase and activate your HTC One by April 30th, 2013. Then you need to take a copy of the proof of purchase, which must show the unique IMEI number of your new HTC One, along with your trade-in and mail it to HTC with a postmark dated no later than May 31st, 2013. The trade-in must be fully functional with no liquid damage or damage to the screen. It also should be deactivated.
With the deadline for registering rapidly approaching, we would suggest you do it now, even if you are not sure about whether or not you are going to pick up the HTC One. It would be quite a bummer to decide to purchase the phone after all, and realize that you don't qualify for the gift card because you never pre-registered.
source: HTC via AndroidGuys