This is where you add your levain and mix it into the dough. I used to also add the salt here, but I started adding the salt a bit later after watching some of the videos from @fullproofbaking.
Bulk fermentation begins at this step, so if you’re taking notes, this is the time to note. I measure the amount of levain on top of the dough (after the Autolyse). Then I wet my hands and spread the levain all over the top of the dough. Next, I take my fingertips and dimple the levain into the dough with my fingertips. The goal of this step is to incorporate the levain throughout your dough. To do this, I use my hand a pull up and stretch over a section of the dough. I then turn the bowl a bit and do this again turning the bowl a bit each time.
Once I’ve gone all the way around the bowl, I use the tips of my fingers to pull up a piece of dough, and squeeze it a bit between my thumb and fingers while pulling gently at the dough. I do this over and over while turning the bowl a bit or moving my hand a bit to get to a new piece of dough. Every few times I do this, I take all four fingers and go under the dough to the middle and pull and stretch the dough gently. This is called the Rubaud method that was invented or popularized by @trevorjaywilson. I do these two things over and over until the dough starts feeling and looking smooth and homogenous.
I cover my bowl at this point and let it rest for 30 minutes. Remember time is magic and does a lot of work for you. It’s good to measure the temperature of your dough at this point and make a note of it.