In a medium jar, combine 100 grams of all-purpose flour with 100 grams of filtered water. Stir well until there are no dry lumps. Cover the jar loosely with a clean cloth or paper towel, securing it with a rubber band.
Let the mixture sit at room temperature (about 70-75°F or 21-24°C) for 24 hours.
On Day 2, add another 100 grams of all-purpose flour and 100 grams of filtered water to the mixture from Day 1. Stir well, cover loosely, and let it sit for another 24 hours.
On Days 3-7, daily discard half of the starter mixture and add 100 grams of all-purpose flour and 100 grams of filtered water. Stir well and cover loosely.
By Day 5 or 6, observe the starter for bubbles forming on the surface and an increase in volume, along with a pleasant tangy odor.
The starter is ready to use when it doubles in size within 4-6 hours of feeding and has a bubbly texture, typically by Day 7.
If you notice any off smells or signs of mold, discard the starter and start the process over.
Once active, you can store the starter in the refrigerator and feed it once a week if not in frequent use.