Day 1 - Mix together 1/4 cup flour and 1/4 cup lukewarm water. Let sit for 2-3 days until bubbles form and it smells of yeast. During this time, stir the mixture whenever you think of it.
Day 4 - Remove half the starter mixture and dump it down the drain. Feed the remaining mixture with 1/4 cup flour and 1/4 cup water. Mix.
Continue to dump and feed exactly the same way every 8-12 hours for 2 weeks or until the mixture reliably doubles in size after feeding.
Store the sourdough starter in the refrigerator until the day before you're ready to make bread. The day before, remove the starter, let it get to room temperature and then feed it. (add 1/4 cup flour and 1/4 cup water) 8-12 hours later, feed it again. It is now ready to use in the sourdough bread recipe of your choice.