Measure out the flours using a food scale. Using a food scale helps to ensure that your getting the proper measurements!
Add water, sourdough starter, salt and flours together in Cambro or large mixing bowl and use your hands to mix the ingredients together. Dough will be “shaggy”, just mix until all the flour is combined. Cover and leave for 15 minutes.
Open Cambro and “stretch and fold” the dough, cover and wait another 15 minutes. Repeat until you have done four total "stretch and folds." Dough should then be smooth and should have lots of stretch by the last stretch.
Cover tightly and leave the dough a room temperature until it has doubled in size (easiest way to measure is put a rubber band around Cambro where dough starts). This can take 6-8 hours in the summer and 10-12 hours in the winter!
Remove dough from Cambro and place on onto lightly floured surface. A granite counter top work great or use a silicone mat. Divide dough in half (should be a little over 900 grams each).
Take each round and pull edges to center all the way around, then flip over and cover with a tea towel for 10 minutes. Do the same thing, pulling edges out a little and tuck into center and flip over. Then take round and gently pull a few inches toward you. You can do this 3 or 4 times and feel the form “tighten” a little. Lift and flip over and place into banneton baskets.
Cover and put into fridge overnight.