He really must have done his homework. Though I guess you can skip a lot of the preflight routine when you don’t plan to land safely.
Very good version of the Q400 with FSX/P3D simulator. I would imagine that’s how he knew to fly it.
Flight simulators mimic pretty much the real thing so it's possible to learn how to get a plane off the ground and maneuver it in the air pretty much on simulators alone which you can run on your own PC. I believe this was how the 9/11 hijackers trained themselves.
Also, there are YouTube videos that show you how to operate just about any type of airplane. I went through a phase where I watched a ton of Cessna 172 videos. I never followed through on taking actual flight lessons but I felt comfortable enough with the basic techniques of flying a Cessna.
Here's a video on how to startup a Q400 turboprop similar to the one we are talking about here. There are countless other videos explaining the general operating of this particular aircraft.