Over on YouTube Adam Łoboda has uploaded a video showing the full steps that he's taken to reverse engineer and clone a wireless garage door key using an RTL-SDR and Arduino.
He starts by using the Universal Radio Hacker software to record a copy of the wireless signal generated by the garage key. Using the software he can then analyze the signal, and determine the preamble data, payload data and pulse width which he can then input into some Arduino code. The Arduino can then generate an identical signal, and transmit it via a cheap FS1000A 433 MHz RF module. Finally, at the end of the video Adam shows the cloned Arduino based garage key working as expected.