Quarter your potatoes and soak in water for 20 minutes.
Either using a food processor or hand blender, pulse the potatoes in the water until the liquid is cloudy and the potatoes reduced to small chunks.
Now you will need to separate the larger potato pieces from the liquid they've been soaking in. You can either pour the mixture through a sieve or strain through a cheesecloth.
Let liquid sit for an hour or so until settled. You'll see a white layer on the very bottom of your container -- that is the starch we're trying to obtain. Pour off the liquid until just the starch is left.
Continue to soak your potato chunks in water and strain until the water runs clear rather than cloudy. This will ensure you get all the starch possible.
Scrape starch out of your container and onto a non-stick surface in a thin layer to dry. You can accelerate the process using a dehydrator or a low-heat oven if necessary.
Once dry, peel off the dried starch and place in a food processor. Pulse until you're left with a fine white powder. Now use in any recipe calling for starch -- food or material!