Measure out the sugar and add it to the blender or spice/coffee grinder. If you're using a grinder, you may need to do this in several batches.
If you're making homemade powdered sugar to store in the pantry for long-term storage, then it's best to add cornstarch (or another starch that is safe to be eaten while raw) to the ground sugar. This will work as an anti-clumping agent. Add 1 tablespoon starch per cup of powdered sugar.
Blend/grind into a fine powder. In a high-speed blender, you may want to pause the machine and scrape down the sides with a spatula to make sure it's all blended equally. This can take as little as 30 seconds up to 3-4 minutes based on the blender size and strength. Using a coffee/spice grinder, the process takes around 30 seconds.
Sift the powdered sugar into a container, repeating the grinding process for the larger pieces until it is all fine. When rubbing a little of the powdered sugar between your fingers, it should feel super fine with very little, or even no grainy texture.
How To Store
Homemade powdered sugar without cornstarch is best used immediately to avoid clumping. However, if starch is added, the sugar can last years when stored in an airtight container.
It's best to use the smallest jug on your blender to make sure the sugar is properly blendable. The larger the jug, the more sugar you'll need to use for it to work properly.
I've tested this process with several sugar varieties, including cane sugar, coconut sugar, and raw turbinado sugar. Varieties with the most molasses content may be a bit too sticky, though (like brown sugar).
If you don't have cornstarch, you can substitute it with another starch; just make sure that it's edible while raw or that you heat treat it first (i.e., arrowroot starch or tapioca starch).
If your blender isn't high-powered, this process can take longer or may not work - unfortunately, I don't have a list of all the blenders/food processors that will and won't work. However, I've always had GREAT results with my coffee/spice grinder, and they are low-cost too!