That's not a question that can be answered in a few words. In general, a portion of the flour, liquid and yeast in the recipe is replaced by some of the sourdough starter, but getting a starter to that point is complex and the timing of the recipe will need to be adjusted as well.
I've written many times about the tradeoff between time and flavor. Sourdough breads to me represent taking that tradeoff to the maximum.
You might look at some of the recipes that KAF has that use starter. That would give you an idea of proportions.
I have a homemade starter (at least 25 years old) that I use in some of their recipes, but I usually need to increase the flour by 1/4 cup for each cup of starter for which the recipe calls. I'm not sure, but I think the KAF starter is less liquidy than mine.
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