Thanks to your help, I can now plane track a video and nicely add a video on top of the plane. However, I have a problem..
So, I only need to track a small part of a large video. I set the range to "Start: 1910, End: 2374" so I could only track the plane for those few hundred frames. And it works!
However, when I use the nodes to add the video in, the video starts playing, on the plane, from frame 0, effectively ending before it even reaches 200 frames.
How can I make it so that the superimposed video starts playing at the frame range I tracked the plane in (starting at frame 1910)? And also, is it possible to loop the video? If so, how?
Here are the nodes I used: