Katakuriko is a fine vegetable starch used as a thickener in Japanese cooking. The starch was traditionally derived from the katakuri plant (trout lily) but now is actually a term describing the more common potato starch.
Combine equal parts of the starch with cold water. Whisk the mixture to combine and create a smooth lump-free paste. Use the paste immediately. You can make a thinner paste using more water to adjust the thickness of the sauce.