Day 1: In a clean glass or plastic container, mix 1/2 cup of flour and 1/2 cup of water until well combined. The mixture should have the consistency of a thick pancake batter. Cover loosely with a cloth and a rubber band or plastic wrap. This will allow air to enter but will keep contaminants and fruit flies out. Place in a spot where you believe the temperature will be best for your sourdough starter. More ideas on this are below.
Days 2-7: Once a day, discard about half of the starter (or transfer it to another container for later use). Add 1/2 cup of flour and 1/2 cup of water to the remaining starter. Stir well to combine. This daily feeding helps establish and strengthen the starter. It provides fresh nutrients for the yeast and bacteria to feed on.
Watch for Activity: As the days progress, you should start to see signs of fermentation. The signs include bubbles forming and the mixture becoming frothy. This indicates that the natural yeast and bacteria in the environment are colonizing and fermenting the starter. You can place a rubber band on the outside of the jar to mark the level of your starter at the beginning of the fermentation window. This will help you see how much it has risen by the next feeding.
Establishing a Feeding Schedule: Feed your starter around the same time each day. By around day 7, your sourdough starter should be active and ready to use. It is now suitable for recipes that call for "sourdough discard". It should have a pleasant sour smell, be bubbly, and double in size within a few hours after feeding. Once you get into a regular baking routine, you can always put your starter "on pause". Do this by placing it in the fridge right after a feed. The lower temperatures will allow you to only feed your starter once per week.