Normally, you set up a sourdough using a Sourdough Starter, some flour and water. You can use yeast too, instead of sourdough starter, but I try to avoid using “pure” yeast, if I can.
The right ratio of starter to flour and water changes with each recipe, as well as the ratio of sourdough to the full dough.
For now, while I flesh out this note more, Marcel Paa has a good overview in his blog post, though it is in German.