Here's what you'll need: old business cards or even index cards or an old deck of playing cards, or even a new deck, I'll be alright with whatever you choose. And you'll also need a pair of scissors.
Cut four even slits on each card, two on one side, two on the other. (Sorry the picture is blurry, I'm still learning about my camera).
After that you can stack as shown below. It's easier than making the card towers that I used to make because this way they won't fall down as easy. Enjoy!