If you have leftover starter from feeding the starter, store it in the freezer. It is a great addition to bread dough, adding more flavor, more strength, and better keeping quality.
To add old starter to a bread dough, use about 16% of the flour contained in the recipe for example, for one loaf use 75 grams starter and also add 1 gram/about 1/8 teaspoon of salt to the total amount of salt.
Defrost the starter and when ready to use, cut it into pieces and let it sit in the water for the recipe (covered) for about 30 minutes to soften the starter so that it integrates evenly into the dough.
Notes: If your starter is a liquid starter, stir in enough flour to form a soft dough and allow it to sit overnight.
If you don’t have a sourdough starter, you can make a Biga:
In a small bowl, whisk together 49 grams of bread flour and 1/16 teaspoon of yeast. With a silicone spatula or wooden spoon, stir in 30 grams/2 tablespoons of water. Continue stirring for 3 to 5 minutes, or until very smooth. The biga should be tacky enough to cling slightly to your fingers.
Cover the bowl tightly with plastic wrap that has been lightly coated with nonstick cooking spray (or place it in a 1 cup food storage container that has been lightly coated with nonstick cooking spray and cover it with a lid). Set it aside until almost doubled in volume (to 3/4 cup) and filled with bubbles. At warm room temperature or in the proofer (80°F/27°C), this will take about 4 to 6 hours. Stir it down. Refrigerate it for 3 days before making the dough.