Well, there are two ways of doing it. Do you want the camera fixed, looking at a moving object, or a moving camera looking at a fixed point, or even both: a moving camera and target.
All it takes is a lot of path_corners. Set "path_corner" value in the trigger_camera to the first one and set it up as you would a train.
For a moving target, it's probably best to use a train, so make a func_train and group it with an origin brush, give it a name and use that name as the camera's looking thingy.