Once your initial mix for the starter has begun to ferment, you'll need to feed it. Feeding gives the yeast more carbohydrates to digest allowing it to continue fermenting.
Uncover the starter and add:
100 grams distilled water or spring water
100 grams unbleached bread flour
Stir the water into the starter to dilute it, then smoothly stir in the flour. Scrape the starter into a clean bowl. Note the date and time of the feeding on the label, then cover the bowl. Here's my starter atop the refrigerator next to what's left of a 50-pound sack of bread flour and a gallon of distilled water:
Now, the waiting game again... If it's warm in the room, the starter should begin fermenting after 6 to 12 hours.
The weather has been cool in New York and I have the windows open so it's not very warm even on the refrigerator. This fed starter began showing signs of active fermentation only after 2 days:
The fine bubbles also indicate that the starter has begun to acidulate: the Lactobacilli that contribute the sour flavor to the bread made from sourdough starter are also working.
Once the fed starter has begun to ferment again, you'll need to feed it again. You can buy some time by stirring down the fermenting starter and letting it come up again, but for a new starter it's best to feed it sooner rather than later.
This time, measure out 100 grams of the starter into a clean bowl and discard the rest. Or if you like, do a second bowl to use as a backup or give away. Feed the starter with 100 grams distilled or spring water and 100 grams unbleached bread flour, smoothly stirring them in. Note the date and time, cover and wait until the starter again shows signs of fermentation. Even if the starter was slow to ferment again after the first feeding it will go more quickly now.
What has happened so far: Simply stated, natural yeast called Saccromyces exiguus inherent in the flour has begun to ferment and live in symbiosis (that just means they can't get along without each other) with the Lactobacilli. S. exiguus is a different species of yeast from S. cerevisiae, the yeast we use for baking. S. cerevisiae isn't capable of living in symbiosis with Lactobacilli which is why you can't use yeast to cultivate a sourdough starter.
Next: Getting the starter ready for baking.