Biga is the leaven for your Sourdough. I am going to make sure it turns out for you, so we will add yeast.
1 teaspoon Yeast
1/4 cup warm Water no hotter then 110 degrees F
3/4 cup Water room temperature
2 1/3 cup Flour All Purpose is fine
1/4 cup Sour Dough Starter
Olive Oil for the bowl
Mix together the warm Water and Yeast let sit until it gets foamy.
Stir in the remaining Water
Add the Flour 1 cup at a time and stir about 3/4 of a minute in between additions.
After the first cup of Flour add the 1/4 cup of Sour Dough Starter.
In a bowl place a small amount of Olive Oil and spread it to the sides.
Add the dough and coat it with the Oil that is in the bowl by turning the Dough over.
Cover with Plastic wrap over the bowl and let stand at room temperature for 1 to 3 days.
The Sourdough is a collection of wild yeast. Yeast grows every where and you may put your starter in the garden, for example. I took mine when I was helping trim the tallest Christmas Tree in the Nation, here in Ferndale, CA. This is the link for the Sourdough starter: