If you need to cut a good amount of it I've found the best way to cut it is stack a few lengths on top of each other and put it on my kapex. Keep the head all the way in and plunge straight down slowly. Once your through you can pull the head of the saw towards you which is essentially cutting backwards. Works like a charm.
We also just use a small Drexel circular saw (3" or whatever it is) with a pvc blade on it.
You can also cut it with a sharp razor blade or a pair of tin snips. It's really not rocket science as the cut goes jnto J-channel on one side and is covered by flashing on the outside edge.