When it comes to setting the mesh on an RC car drivetrain, you don't need paper or any other trick; you just need to know what you're trying to do: get the spur and pinion gear to fit as close together as possible while still having a tiny amount of play.
This ensures things are low-friction, and won't skip teeth.
Just make sure you're holding that pinion gear firmly when you test your spur's ability to rock back and forth. One of the gears needs to be held rigidly as you check the ability of the other to move, and the pinion is typically the best choice, as the motor shaft may not be able to freely rotate depending on the motor's internal construction.
For the best feel, hold the pinion, check the spur, just like in the video.
