Place your mixing bowl on the scale and weigh 700g of the water (save 50g) and 200g of your leaven. Using your hand, incorporate the leaven a bit by squeezing it through your fingers. Save the rest of your leaven - this is now your starter! Feed it as usual.
Add both flours and mix together with your hand. Mix until you don't see any more dry flour in the bowl. The dough will become sticky, so it's useful to keep one hand clean. Dunk your clean hand in water, and remove the sticky dough from your other hand. Then dip your dough spatula in water and scrape the edges of the bowl, making it as clean as possible.
Cover the bowl with a clean kitchen towel or plastic and let the dough rest for 30 minutes. This is called the autolyse, which allows the flours to absorb the water, activating the enzymes which begins the gluten development. This step is critical and cannot be rushed.
After the autolyse, sprinkle the salt over the dough and then add the remaining 50g of water. Poke your fingers into the dough to press some salt deep inside, then fold over itself about a dozen times or so to incorporate the salt. Cover the bowl again; the bulk fermentation has begun. Set your timer for 30 minutes.
The bulk fermentation takes four hours. During the first two hours of the bulk fermentation, the dough must be folded four times, or every 30 minutes. This is similar to kneading but is much more gentle to preserve the natural gases that become captured in the dough, and is much easier on the baker.
To fold the dough, first imagine your bowl as a compass: the edge furthest from you is north, the right edge is east, the closest edge to you is south, and the left edge is west. Dip your hand in water and reach under the dough at the east point. Grabbing it gently but firmly, pull the dough out to the east and then fold it over itself toward the west. Rotate the bowl a quarter-turn, and repeat. Do this for each "corner" of your compass, then cover with a kitchen towel.
Set your timer for 30 minutes, and repeat the process three more times.
Your dough now gets to rest, covered and untouched, for 1 1/2 - 2 hours. During this time, flavor and strength is developed, so don't rush this step.
After the bulk fermentation, pull all the dough onto a floured work surface using a dough spatula. With your bench knife, cut the dough into two or three even pieces (depending how many loaves you want, and how big you want them). Scrape the bench knife under one piece, and move it away from the other(s).
The pieces now need to be pre-shaped. Working with one piece at a time, pull the west side of the dough out and fold it toward the east. Then pull the east side out and fold it over and toward the west. Rotate your dough 90°, and repeat. Do the same for the remaining piece(s), then lightly dust with flour and cover with a kitchen towel. This prevents a skin from forming on the outside of the dough. Let rest for 30 minutes. This is called the bench rest.
For the final shaping, care must be taken not to deflate the dough. Gently rub off any excess flour - the top of the dough will become the inside of the loaf, so you don't want any extra flour inside. Just as in the pre-shaping, pull the west side of the dough out and fold it toward the east. Then pull the east side out and fold it over and toward the west. Rotate your dough 90°, and repeat. Flip the loaf so it is seam side down on your work surface, and using both hands, twist the dough as you tuck it under itself. There are great YouTube videos with different techniques for this, but the goal is the same: to increase the surface tension without tearing the dough. You'll feel the dough tighten as you do this. Repeat for the remaining piece(s).
Line your proofing baskets or medium-size bowls with basket liners or clean linen kitchen towels. Lightly dust them with rice flour (all-purpose works if you don't have rice flour, just not as well), covering the sides and bottom. This prevents the dough from sticking when you flip it out. Lift each piece of dough with the bench knife and flip it gently into the basket, so the seams are facing up.
Now begins the final rise. You can cover the loaves and leave them on the counter for 3-4 hours if you'd like to bake today. However, what I recommend is using your refrigerator to slow the final rise so you can bake in the morning. This is called cold-proofing. To do this, slide each basket into their own plastic grocery or produce bag, and place in the fridge overnight. The plastic is used to prevent fridge odors from absorbing into the dough. The dough will slowly rise over 8-12 hours.