About an hour before the levain is ready, combine the flour and water in a large bowl. Use wet hands to mix until no dry bits remain. Cover with a plate or damp towel and rest for 30 to 60 minutes.
Add all the levain and salt. Mix with wet hands until the dough is sticky and roughly combined. Give it a few slap and fold kneads in the bowl to begin creating structure. This is when you lift the dough slightly and slap it back down into the bowl, folding it over itself. After this, cover the bowl and let the dough rest for 20 minutes.
Over the next 3 hours, perform stretch and folds (or coil folds) every 30 minutes to build strength. Keep your hands damp to prevent sticking. It’s 5 to 6 sets total. After each fold, cover the dough and let it rest until the next one.
After the final fold, transfer the dough to a clean bowl and cover it to finish fermenting.
Let it rise in a warm spot until it looks puffy, jiggles slightly when you move the bowl, and has grown about 40-50% (not doubled). This might take between 1-3 hours, depending on your room temperature, so watch the dough, not the clock.
Tip: Ideal bulk fermentation temperature is around 24 °C (75 °F). Cooler kitchens take longer, warmer ones the dough will rise faster.