Put 1 Tbsp potato starch in a cup and fill the rest with cane sugar. Put that in a blender and pulse it until you have a fine powder. Don’t run it continuously or you will burn out your blender and the sugar will cake onto the blade. Shake the container around every now and then. This can take quite a while, so patience is key here. The consistency is usually still a bit grainy.
This can also be done in a coffee grinder but needs to be done in smaller batches. You can get a finer grind in a coffee mill.