Day 1: In a glass jar, mix 100g whole wheat flour and 100g water. Stir well with a wooden spoon. Cover loosely and let sit at room temperature for 24 hours.
Day 2: Discard half of the mixture. Add 100g all-purpose flour and 100g water. Stir well, cover loosely, and let sit at room temperature for another 24 hours.
Repeat the Day 2 process for 5-7 days, until the starter is bubbly and has a pleasant sour smell.