Sourdough is made of two parts of flour, and a volume of water. The covered mixture is allowed to spontaneously ferment for a certain time at a certain temperature. Flour contains naturally lactic acid bacteria. Which will develop and acidify the mixture to leaven.
The next day, add in a quarter of the leaven again two parts flour and one part water. Repeat this operation for 3 to 5 days, then you have your own starter made ​​from the wild yeasts that float around in the kitchen and the microbial flora of the flour.
Do not use metal (except stainless steel), because the lactic acid will affect the pot. That is bad for the pot, the yeast and the consumer.

You never know whether you 've got the good yeasts. You should try. The mixture should be acidic, but not smell bad. You may see no mold. Addition of some sweetener or lactic acid bacteria may also be helpful.
Some recipes also add some baker's yeast. Or some white wine, yogurt or buttermilk. You can always get another yeast or leaven.

The French always keep an old piece of dough from the previous day. There is then water and flour added to. A French levain always contains a quantity of yeast.