Sourdough bread starter is made via a process called "feeding" which involves mixing a portion of the starter with flour and water before allowing it to rise and bubble. The starter is then added to the dough once it's ready.
If using a homemade starter from a recipe, check its readiness by floating some of it in a container of room-temperature water—if the small piece floats, your sourdough starter should be ready for baking. If it sinks, you may have to let the starter sit for a bit longer for the dough to rise.