You'll need a bunch of parts for this project! You can noodle around with most of the parts. If you want to fit all the parts in the Lockitron enclosure, you'll also want a 500mA battery (Some experimentation may be required to get a 500mA battery that fits). The battery may have to go outside the box!
Since this isn't a beginner tutorial, please familiarize yourself with the usage of the Metro Mini (make sure you can upload code from the Arduino IDE, check the serial monitor) and FONA 800 (get your SIM card, verify you can use the FONA with the example Arduino library and examples to connect to the cellular network and send an SMS, etc) and TB6612 motor driver.