This is how to make a new sourdough starter from scratch, using just filtered water and unbleached flour.
Day 0: Mix 1/2 cup water (113 grams) with just under a cup (3/4 cup + 2 Tablespoons or 113 grams) flour, stir and cover with a loose lid or cloth.
Day 1 through 4: Feed once a day by discarding all but 4 ounces of starter and adding that 113 grams each of flour and water, stirring to combine, cover.
Day 5 through 8 (or more, until bubbly and starter doubles in size): Begin twice a day feedings, keeping the same 4 ounces of starter, adding 113 grams of flour and water and stirring to combine, cover.
Maintenance - transfer a bubbly starter (a few hours after feeding) to your refrigerator, and take out to feed once a week, using the same process as above. Keep 4 ounces of starter, discard the rest, stir together 113 grams each of flour and water.
Of you see a brownish liquid on top, this is called "hooch." This means it is time to feed your starter. You can either pour it off or stir it in, and then feed it.
Your starter may smell a little funky as you are establishing it. Just make sure you are feeding it regularly, and the smell should start to smell nicer. I noticed a weird smell around day 5. When I started two-a-day feeding, it went away.
Alternative flour options (for starting, or feeding existing starter):