Mix flour with water and then cover well with foil and leave at 25-28 °C for one day (better two days).
Mix the ingredients of the second stage with the previous batch (1st stage). The first batch already smells pleasantly sour. Cover the whole mixture again and let it stand at 25-28 °C for another 24 hours.
Now mix the ingredients of the third stage with the previous preparations (stages 1 and 2) and let stand covered again for one day at about 25-28 °C.
Put the unused sourdough in a screw jar and store in the refrigerator.
The next time you bake, the sourdough preparation will be much faster with the rest (the stored 100 g).