Three INCHES. ignition in ON position (does not need to be running)
http://youtu.be/zqlox4LYmEs
This was the exact video that got me going
http://youtu.be/qSe_Yhez-ro
This one appears to be an official tutorial. Has additional details depending on which type of garage opener you have.
The KEY to this working is understanding the concept. It's a two part process
➀ train the car to pretend it's the hand remote
➁ add the car to the authorized remotes