Flynn left the Packers in 2011 following four years of playing backup to starter Aaron Rodgers, and went on to have spells with the Seattle Seahawks, Oakland Raiders and Buffalo Bills.
Failure to secure a starting gig meant that he was available to sign on and help the Packers, who lost Rodgers recently to a shoulder injury.
"Leaving Green Bay definitely didn't go the way I planned," Flynn said to Packers.com. "Since I left here, I've learned three different playbooks, three different terminologies, three different philosophies on how to attack defense, how to do footwork this way or that way.
"But I've been ingrained in this Packers offense for so long. I knew it inside and out, was fluent in the language, fluent in what we're trying to do, that it's come back pretty quickly, and I see myself having it down pretty close to 100 percent very soon."
The Packers currently sit in third in the NFC North, but still maintain hopes of a playoff place.