After repeated threats to my physical wellbeing and a bit of wheedling by rocketscientist2105, i was finally persuaded to put a link to his orangeboard.
Step 1: Select the Steel
Tools are made from hard steel, right? They usually are.
If the tool you're using has to put up with a lot of bending and twisting and hammering and general abuse, it'll probably be a meduim carbon steel, or a spring steel. Either of these is fine, in fact you could just use a bit of a car spring like jtobako did, that'll do the job perfectly, because throwing knives need to be flexible. If the tool you're using has to be hard and wear-resistant, like a file or a drill bit (I don't recommend using drill bits.) then it'll most likely be high carbon steel. I used a file. The first thing we need to think about when forging files is that they have teeth. Forge them with the teeth still on and you'll trap scale in the gaps, then fold the teeth over. it'll be terrible. Grind the teeth off the file. it helps if you anneal the steel before you do this. To anneal steel, get a good fire going and chuck the steel in. Let the whole piece get to a dull red heat and then let the fire die off slowly. If you anneal the file, you won't wear away your grinding wheel too quickly.
Step 2: Start Forging...or Not.
I looked up the naruto kunai on the interweb and its a lot different to the normal Japanese kunai (intended as a tool, like a trowel, or a wrecking bar, not a weapon) it LOOKS alright, with a triangular dagger blade and a ring on the handle. I couldn't be bothered to shape the steel into the triangly-shape, and i'm not going to learn to forge weld, just to get the loop on the end closed, so i'll add my own little features that'll make it much easier for me to forge it, but not detract from its functionality.
The blade will be a knife blade; only one side sharp, and the loop will be open-ish. In the next step, i'll show you how i made the loop.
Step 3: Make the Loop
For the next step of the bendening, you'll need to make a quwstion-markish sort of shape, by bending it around again.
Now comes the tricky part, you'll need to get inside the flat bit and bend that over too. My best advise is Be Creative.
Finally in this step, finish the bendification by pushing the end of the steel over to meet the main part of the bar. After that, I hot-cut off the rest of the file, that I hadn't ground so it was smaller and easier to work with. Now its about 6 inches long with a loop on the end.
Step 4: Forge the Blade
Forge the blade by heating it up, holding it at an angle to the anvil and hitting it to create a blade shape. jtobako's got a much better instructable on making the bevel. Once the shape of the knife is made, let it cool slowly, and i mean really slowly. Hang it in the air if you can, or put it on some sand. If you found the meaning in my appalling description and put it into practice, you should have something that looks like this.
Step 5: Grind It to Shape
If you want to do it by hand, use a vice and a file. Have fun in your toils, I wish you well! Personally, I believe that tools are made to be used. That's why i'm using my bench grinder to take away all the excess steel. This beast bites deep and does it quickly. One moment's lack of attention could have you looking on the floor for your fingers or, even worse, could completely mess up your knife! porceed with caution. Once it's been ground to shape, you should set about removing the grind lines. THIS is where I recommend a file, and a fine one at that. Again, proceed carefully. After the file, I moved on to a coarse waterstone.
I know for a fact that the best way to proceed is to get most, if not all of the necessary shaping and grinding out of the way before heat-treating the steel, I mean, why make extra work for yourself?
Step 6: Heat-treat the Blade
All hardened blades need tempering, but some more than others. Throwing knives need to be very tough, because they smash into things with great force. If it cracks, it's an embarassment to the person using it, and to the person who made it.
So, warm up the steel with a blowtorch, or in your forge, it doesn't matter, as long as it gets hot enough. There is a point at which the steel's internal structure changes from the soft crystals called Ferrite into Austensite. This is the point at which the steel is Quenched. (luckily, as the steel changes its crystal form, it stops being magnetic, that makes things a lot easier, doesn't it?) Quenching the steel cools it rapidly and changes the Austensite into Martensite. By quenching the steel, we are imparting tremendous stress to its crystals, this is what makes hardened steel hard. But it's brittle, too, so what are we to do?
Temper it. Change some of the Martensite into Troosite.
To temper the hardened steel, you need to warm it up again, but not as much this time. I suggest that you either do it slowly with a blowtorch, or, if you're using a coal forge, that you heat up a big lump of steel ot a dull red heat and let the knife draw heat out of the lump and into itself. You're looking for an all-over blue temper on this one. Don't let it get too hot, otherwise that lovely blue colour will lose its brilliance and the knife will lose its toughness. A quick note: Steels with more carbon in them need tempering slightly more for the same effect. Experience is the only real teacher here.
Step 7: Polish and Test the Blade
But there's one more thing. You have a knife, but its time to have some fun. Got a shed? some cardboard boxes? irritating neighbours?
I think you know what to do.
Thanks for getting to the end of this, don't hesitate to ask me any questions. I know I could have been clearer on most of the points I made.