Sourdough is nothing more than wild yeast that is all around us. You need to give it time and nutrients to start, and then feed it regularly by adding the same amount of flour and water every week or two. Opt for 50 g of flour on 50 ml of water, or 100 g of flour on 50 ml of water, depends on how much starter you use and need to replenish.
Storing your starter is easy. Just keep it in a fridge, take a portion you need for your bread and leave the rest chilled. Every 7-10 days add another equal amount of flour and water (50 g + 50 ml or 100 g + 100 ml), mix thoroughly and put back to the fridge. It will be happy for another week or so.
I keep my starter in a bail jar. I cover the lid, but without closing the clasp. This way it receives enough air to live, but at the same time it is protected from drying out.
Healthy starter should smell acidic, somewhere between lemony and beery. When you are not sure if your starter is good just by looking or smelling it (although you really should be), taste a small drop. It should taste acidic, but not bitter. If there is any trace of mould on top of it, discard it and start again.
If there is a dry layer on the surface and the starter seems fresh underneath, it is all right to just get rid of the dry part and keep the rest.
✽ 100 g of all-purpose flour
✽ 100 ml of water
It is enough to mix the same quantity of flour and water and leave out on a kitchen counter lightly covered with a tea towel or a paper napkin. It is good for 14 hours, then it is time to feed it.
Feed your new yeast by adding 100 g of flour and 100 ml of water every day for seven days.
On day 2 or 3, you should be able to observe first bubbles and the mixture will start smelling sour. It will become a little more liquid, but should not change its colour. When you mix in fresh flour and water, the bubbles will disappear, but they should re-appear about an hour after each feeding. Sometimes clear liquid will separate on the surface, it is completely normal. Either mix it in or discard it, as you wish.
After seven days your starter should be ready, but it can still be a little weak. You can give it one week (the first week without feeding) to get stronger and then start baking your delicious bread.