Matt...
...the answer is simple, but not easy (oh, by the way, this is a serious answer

).
You start off with a book about card magic (or general magic if you don't intend to specialise) and start learning. Practice a lot and then practice some more.
After you have learned and performed a few of the classic tricks and understand the basic principles of card magic (or general magic) you'll be on the way to being in a position to understand the card through window effect.
If, on the other hand, you've seen it done and just want to know how... personally I suggest you
don't learn how magic is performed. Trust me, once you get into learning the "how" of magic, the joy of watching it performed changes dramatically. You'll go from the pure "wow" and enjoyment of a spectator to a more technical appreciation which isn't for most people.
Either way, best of luck (and like Mandrake said, methods to tricks aren't given out on here).