In a clean glass jar or container, combine the flour and warm water. Mix thoroughly until a thick, smooth batter forms. Cover the jar loosely with a cloth or paper towel to allow airflow while preventing dust and insects from getting in. Place the jar in a warm, dark spot (around 70-75°F or 21-24°C). Let it ferment for 3 to 7 days. Every 24 hours, feed the starter by discarding half of it and adding 1 tablespoon of flour and 1 tablespoon of warm water. Stir well and keep covered. After several days of fermentation, bubbles will appear, and the mixture will develop a sour aroma, indicating the yeast is active and ready to use.