Perform a float test on the levain to make sure it is ready to use. If it's not ready, try again in 30 minutes.
Your water and flour mixture should now be soft and stretchy. Add 50g of the levain into the center and incorporate by pinching and folding over the dough. Add the salt and water and incorporate by pinching and folding again. Let sit for 10 minutes.
Knead your dough. Pour the dough onto a clean countertop and knead using the slap and fold method for about 10 minutes. It will be very wet and sticky at first. Continue to slap and fold and resist the urge to add flour. After a while, the dough will stick to itself more than to your hands or the countertop. Perform a window pane test to see if gluten has developed sufficiently. Return the dough to the bowl and cover.
Allow your dough to bulk ferment. Let your dough rest anywhere from 3 - 8 hours depending on the temperature of your kitchen and the activity of your starter. Once every hour during the bulk ferment, fold and turn your dough. To do so, grab the edge of the dough and pull up until it has stretched out fully, the bring down to the center. Do this with all 4 edges. At the end of the bulk ferment, the dough should feel light and airy with visible bubbles on the top and sides.
Shape your dough. Pour your dough onto a lightly floured surface. Grab the edges of the dough and pinch them in the center of the dough so you have a roughly round package. Roll your dough over and let it rest on this seam for 10 minutes. This helps the gluten relax into the final shape. Then, using your hands or a bench scraper, shape into a sphere and let rest for about a minute.
Line a round bowl with floured kitchen or paper towels. Smoothly transfer your dough into the bowl, with the seam side facing up. Proof for 45 minutes, then cover and place in fridge and finish the proof overnight.