Most sourdough recipes call for "fresh" sourdough starter. The term fresh refers to sourdough starter where the yeast and bacteria which comprise the starter are in an optimal state of activity and ready to leaven baked goods.
If your sourdough starter is normally kept in the refrigerator, here are the steps for making fresh sourdough starter prior to baking:
Remove 1/4 cup of sourdough starter from the fridge and place it in a large jar or bowl. Feed the starter using one of the following methods:
If using a scale, combine equal amounts of sourdough starter, flour and water by weight. For example, feed 50 grams of sourdough starter with 50 grams of flour and 50 grams of water.
If using measuring cups, one part starter should be fed with one part water and a scant two parts flour. For example, feed 1/4 cup of starter with 1/4 cup of water and a scant 1/2 cup of flour.
Mix the starter, flour and water together being sure to incorporate plenty of air. Cover the starter with a loose lid, towel, etc. (so the naturally created gas can escape). Leave in a warm spot for 4-12 hours until the starter becomes bubbly.
Repeat this process two more times prior to baking using the same ratios prescribed above. To avoid making too much sourdough starter, discard some starter before each feeding or use excess starter to make Sourdough Pancakes.
By the third feeding the starter should be very bubbly and rising to double its size within 4-8 hours of being fed. This indicates the yeast and bacteria are creating adequate gas to properly leaven your bread.