Place a clean quart-sized container like a glass bowl or a mason jar on a digital kitchen scale, and tare the weight (set it to zero).
Add 100 grams whole wheat flour and 100 ml (equals 100 grams) warm (80°F/27°C) water to container. Stir until no dry flour can be seen.
Cover top with a breathable cloth or paper towel, and secure with the metal ring or a rubber band. Place in a location where the temperature is at least 70°F/21°C.
Day 2
Pour out all except 100 grams (about ½ cup) of starter. You'll remove 100 grams or about ½ cup). You can keep a separate larger container for this “discarded” starter, which can be used for many other baking purposes.
Add 100 grams all-purpose flour and 100 ml warm water to the starter. Stir well until no dry flour can be seen. Store in its warm spot again for 24 hours.
Days 3 to ? (7 to 21 days)
Discard all except 100 grams of the starter.
Add 100 grams all purpose flour and 100 ml water to the starter. Stir well.
By the third day, your starter should start to be bubbly and smell slightly sour. Keep going with daily feedings (repeating steps 1 & 2) until starter is vigorously bubbling and smells yeasty.
When a starter is consistently doubling in volume within six to eight hours after a feeding, it is healthy and has a well-established culture. Note that this may take a few more days or even a few weeks.