All you have to do is go to the Motorola signup form (link below), and use the promo code "todayshow50" when signing up. Motorola will then send you a unique code that you can use when buying your Moto X through the Moto Maker site. It looks like the promotion is good for all carriers, but the deal can't be used on a Developer Edition; and, you cannot use this deal to order a bamboo Moto X, those are still full price plus $100.
The deal also states that you will get 50% off "an accessory", which means just one accessory you choose., but again there are limitations there. The 50% off an accessory deal does not include the "Chromecast, Roadster Pro, Moto G Case and Shells, JBL Micro Wireless, Micro USB Power Cable, DECK and TRACKS AIR by SOL REPUBLIC x Motorola products".
Registration for the promo codes will be open until 8PM EST December 21st or whenever supplies run out. Once you get your code from Motorola, you'll have 4 days to make a purchase before your code expires.