They started fermenting a flour and water mixture by using yeast. The Egyptians also developed ovens in which several loaves of bread could be baked at the same time. Bakers started using yeast to improve the rising action. Also fermented dough (sourdough) was added  to flour to make bread baked. That method was especially popular in Slavic lands. To prepare sourdough leaven you need some basic ingredients like water and the whole-meal flour. Mix flour and warm water. Place the dough in a bowl and let it sit in a worm ( about  30^o C ) space for 36 hours. Meanwhile, the lactic fermentation starts. This fermented  bread starter is basic for bread baking. To prepare sourdough, people also used to add dough from the previous baking.
 

This is where people were making bread