This is the simplest preferment of them all. You merely take a portion of fully developed dough off the counter before shaping the loaves and save it in a covered container or the refrigerator for use in the next batch of dough. (Some say you can freeze the old dough, but I’ve not had good luck freezing old dough and then using it.) If you are making the same bread the next time, your old dough will have all the ingredients in the proper proportions, so you don’t need to make any special allowances for it in computing the ingredients; it’s just like adding another bit of dough. If you are making a different dough, then you will need to take into consideration the ingredients in the old dough.
How much to take out? I use somewhere between 10 and 20%, but that’s a matter of personal preference. I rarely make the same bread on two successive bakes, so I rarely use this method, but it’s good to know that it is available if the need arises.