Uchigumori powder is often used by sword sharpeners for extra fine natural stone finish in the final stages of polishing. This is about 4 ounces worth of powder and it's traditionally used with a piece of cheesecloth to sift the stone powder from the larger particles. So to use place some powder into a square handkerchief and tie it at the top and then tap the balled powder against the knife to apply. You can use this mixed with a drop of water to create a little slurry on the blade while you work if you like.
It also makes an excellent fine mud for your hard finishing stones. Use the same technique to dust the stone and then and a few drops of water before you begin sharpening.