I first saw about these puzzles on the Twitter page of one of my favorite authors:
Dave Richeson, the writer of the "Tales of Impossibility". Then, I searched more and actually found the puzzle on M+A+T+H=Love's blog (One of the best resources on the web about teaching math). They are called "crazy eights".
All you need to do is print the pages back to back and cut from the indicated lines. You may even get a blank paper and write the numbers on both sides. Then the goal is to collect all the same kinds of numbers together.
I liked the idea so I searched more for similar puzzles. Here is Wolfram's page about Tetraflexagons that can be turned into paper puzzles.
The green one, which includes only 1,2, and 3 is pretty easy. Then the rest has different challenge levels for different numbers.
Here are the templates I have used