Add flour to a quart glass mason jar.

If using, add the dried sourdough starter.  (This ensures it will capture the right wild yeasts.)

Add just over half as much water as you did flour -- about 1/4 cup, plus a little extra as needed to make a thick batter.

Cover it with a clean cloth and a rubber band.

Place it in a warm (but not too hot) place.  When it's on, the stove is too hot (over 120ºF will kill it), but after dinner, when it's warm is good.  It should ideally be between 65ºF and 85ºF.

For the first three days, you must feed your starter every 12 hours or twice daily (using the same flour and water as you fed it to start).  Throw out half the starter before feeding so the volume stays around 1 cup.  Too much volume now, and it won't have enough bacteria to rise properly.

Once the starter is bubbling and doubling in volume 4 - 5 hours after a feeding, it is ready to bake bread!