It's pretty easy actually, you just need the Animation feature open (you CAN do it without, but it's much more difficult). Under the camera settings you can create keyframes on certain frames located at the bottom of the program screen. For example, if you want to make a flyover that lasts 100 frames (that's only 4 seconds if the video runs at 24fps), you'd put the starting location on the first frame, then, on the final frame, you'd set a new keyframe for where you want the camera to be. Terragen will automatically render the frames between. Then I just import the images into a program and set the FPS, then export as a movie.
Not sure if that made sense... but I picked it up very quickly without much help. Planetside was nice and kept animation quite basic
