Beat air into flour, water in mason jar until it’s mixed well without any dried bits.
Place lid on jar without tightening, allow air flow. Keep in a warmish place, between 70°F and 85ºF. I find my kitchen counter is perfect. Top of fridge or in oven with light on are also good options! You should see bubbles in 4-8 hours.
If no bubbles or starter appear weak, discard 80% of the old starter and add another cup of flour and another cup of water. Stir well and cover.
Perform a few feedings in 8-10 hour cycles.
Depending on conditions, it could take from 3-8 days to build a new starter from scratch.
Once your starter is bubbly and double the original size, it is ready to use!
Storage: Add ¼ cup of flour and water, mix before putting in to the fridge. To reactivate from fridge, discard and feed.
OR If you bake more than twice a week, consider keeping the starter out on the counter. Feed daily.