Finally a kunai made of paper (and a pencil)
Paper (rectangle, not square)
Pencil (new and unsharpened)
Tape (duct tape recomended)
This is pretty simple and requires little origami knowledge.
Step 1: Step one
Take a regular piece of paper and fold it in half long ways(hot dog style as the little ones would call it) and unfold so you have a nice crease. Then fold the edges in so you get a piece that looks close to the image.
Step 2: Fold in half
Fold over the top towards you (hamburger style...) so it looks like the pictures.
Step 3: Step 3
Remember those paper airplanes you used to (and maybe you still do) make? Do something like that here. Also it will help for later if you crease a line underneath the triangle part of the first picture.
A suggestion by a certain commenter says that you should crease everything as much as you can after this step. I'm sure it will work
Step 4: Step 4
Now using the line you hopefully just creased, fold over a part of it so the folded part touches the crease line in step 3, the middle, and the edge of the left side. In the picture, it looks like it doesn't touch the side but it does. Its just folding up. Unfold and do the same with the other side.
Step 5: Kind of Complicated...
Now you should fold the bottom edges in like the first picture. Make sure they line up with the middle. After that, use the crease that you made in step 4 to determine where to refold the edges out They should line up with the crease like the picture (which is a wee bit dark. Sorry...)
Step 6: Get it to look like the kunai
Unfold the last fold you just made so it looks like the first photo from step five except with a crease. Fold over the flaps on that crease so one flap goes on the other.
Step 7: It's gonna kill you!
Ok, now you finished it. Or so you thought. Now you have to do the same thing again. Sorry!