To make this shuriken, you need 8 pieces of square paper. Post-it notes work fine.
Sorry my pictures are pretty blurry.
Step 1: Making the first few pieces
1) Fold it in half lengthwise, then unfold.
2) Fold two adjacent corners that are on opposite sides of the first fold to the center.
3) (This step is hard to explain, so just follow the picture.) Fold the paper in half along the first fold and push down the back to make a parallelogram.
Then take another post-it and repeat the above steps.
Next, fit the front of the second piece into the back of the first piece like in the 6th picture. Then, fold the two protruding parts of the back of the first piece into the slit on the second piece.
Step 2: Making the rest
Make 6 more pieces and attack them as you did the first 2. Fitting the last one in is sort of hard, but you're on your own for that part as I can't help you.
Now, you should have an octagon with an octagon-shaped hole in the center. Push in on the sides and the octagon will collapse, turning into an 8-sided shuriken.
Final words of wisdom: If you are punished/grounded/spanked/whipped/sued/killed for inflicting pain/causing injury/killing someone/something with this shuriken/shaken...well, that's your problem.