Throwing knives are used in specific martial arts disciplines as well as for some circus shows and sport exhibitions. These knives aren't constructed the same as your hunting or pocket knife, and have special attributes that allow one to throw the knife accurately such as a specific weight, length and blade shape. Throwing knives should be sharpened after every use to keep the blade at an optimal performance level. Since most typical throwing knives have a long double edged blade, a slot sharpener is the best tool to use.
Place the slot sharpener on a flat stable surface and hold it in place with your hand.
Position the blade in the "V" slot, closest to the knife handle.
Apply light pressure and pull the blade through the slot. Repeat this process four times, then gently thumb the blade to ensure that you've sharpened the edge to your satisfaction.
Turn the blade to the other side, then repeat the process above.
You can also use a whetstone or honing oil if you prefer to sharpen and keep your blade in peak condition.
Use extreme caution when sharpening knives. As you hold the sharpener, ensure that your fingers are not in the way of the knife as you run it through the slot. This can easily cut your skin.