Step 1: In a clean liter-sized glass jar, combine 100g unbleached all-purpose flour, 50g plain instant potato flakes, and 150g warm filtered water (80-85°F / 27-29°C).
Step 2: Mix thoroughly with a non-metallic spoon until no dry pockets of flour remain. The mixture will be thick.
Step 3: Cover the jar loosely with a lid, plastic wrap, or a clean kitchen towel to allow air exchange. Secure with a rubber band if needed.
Step 4: Place the starter in a warm spot (70-75°F / 21-24°C) away from direct sunlight for 24 hours.
Step 5: After 24 hours, discard all but 100g of your starter. To the remaining 100g, add 100g unbleached all-purpose flour and 100g warm filtered water. Mix well until smooth.
Step 6: Cover loosely and return to its warm spot for another 24 hours.
Step 7: Repeat the discarding and feeding process (discard all but 100g, feed with 100g flour and 100g water) daily for 5-6 more days.
Step 8: By day 7, your ultimate potato flake sourdough starter should be consistently doubling in size within 4-8 hours after feeding, be full of bubbles, and smell pleasantly yeasty and sour. It should pass the 'float test' (a small spoonful floats in water).