Step 1

To begin: Mix together 125 grams flour and 125 grams water with a clean hand in a medium glass bowl. Cover the bowl with a tea towel and let sit undisturbed at room temperature until the mixture is full of bubbles and has nearly doubled in size, usually 2 to 3 days. During this time, yeasts and bacteria from the air and from the flour — and probably from you — will set up housekeeping in the bowl.

Step 2

For daily feeding: Peel back any crust that may have formed and transfer 20% of the culture (50 grams) to a clean, wide-mouthed jar. Stir in 100 grams flour and 100 grams water, loosely screw on the lid, and stash at room temperature for 24 hours. (The culture will have a stinky-sour smell at this point.) Discard the rest of the original mixture.

Step 3

Repeat step 2 every 24 hours for 5 days. By then, the culture should smell yeasty-sweet-sour, which means you're ready to put the starter to work.