A sourdough starter is a valuable tool and the knowledge of how to make one can help you out in unusual situations (like not being able to get yeast from the store.)
In a glass jar, mix together the flour and water until no lumps remain. Scrape down the sides and cover with plastic wrap. Mark on the glass jar where the top of the starter is, so you can visually see when it rises. Leave on the counter at room temperature for 2-3 days.
After it has bubbled and activity is present by it rising and deflating, discard everything but 1/4 cup. Add in 1/2 cup flour and 1/4 cup water and stir until no lumps remain. Allow to sit at room temperature for 24 hours.
Add 1/2 cup of the starter to a new jar and mix in 1/2 cup water and 1/2 cup flour. Every 24 hours, feed the starter in a ratio of 1:1:1. You will mix together 1/2 cup starter, 1/2 cup flour, and 1/2 cup water. The starter will begin to grow and increase the number of bubbles it is producing. Once the starter begins to double in 12 hours, begin feeding it once every 12 hours. Maintain this feeding schedule for 3-5 days. Store at room temperature.
Continue to feed the starter in a 1:1:1 ratio daily. When the sourdough starter doubles 5-6 hours after feeding, you can begin using it brownie, pancakes, cake, and muffin recipes. I would wait for at least 30 days of feeding to give an attempt at bread.
You can store the sourdough starter in the fridge and feed it once a week. Make sure to let it come to room temperature before feeding. If you bake often, you can keep it on the counter and continue to feed it daily.