In a clean jar, mix 50 g unbleached flour and 50 g room-temperature filtered water until no dry flour remains.
Cover loosely, mark the level, and leave at room temperature for 24 hours.
Every 24 hours, keep 25 g starter and discard the rest.
Add 50 g flour and 50 g water.
Mix, cover loosely, and mark the level.
If it seems too runny, use 45 g water at the next feeding.
Once it reliably doubles within 4–8 hours after feeding, for three feedings in a row, it’s ready to bake with.