For a strong and ready-to-use sourdough starter, make sure to feed it 2-3 times before use. The starter will be really good to use when lots of bubbles appear in it. Feed it with an equal amount of water and flour mixture.
Pour the water into the mixer, add the flour, and mix on medium speed until the dough is combined. When a uniform dough is formed, cover, and let it rest for half an hour.
Add the sourdough starter and mix on low speed for 5 minutes until the sourdough is absorbed into the dough. Cover and leave for 15 minutes.
Add the salt and kneed for 3 minutes. After 3 minutes add the olive oil and increase to medium speed. Stir until the oil is completely absorbed into the dough, the dough is uniform and the gluten is strong. It can also take 10 minutes. Make sure that the dough won't get warm.
Transfer the dough to proof in a bowl or greased container. The first proof is for 40 minutes.
After 40 minutes start the first fold. Grasp the end of the dough, stretch upwards, and fold into the center of the dough while turning the bowl. Fold between 6-7 such folds from the edges of the dough to the center. Let proof again for another 40 minutes.
Repeat the folds, and proof again for 40 minutes.
After the dough has reached the desired volume, flour the surface and transfer the dough over the flour. Using a dough cutter, divide into 200-gram pieces, arrange them into a circle or bun, and place on a greased baking tray. If you want to use only part of the dough you can freeze some of it now.