On the first day, mix flour and water into a homogeneous mass without lumps. I mixed it in a canning jar from IKEA, but it just needs to be a large jar where the lid can sit loosely on top.

Let the sourdough sit at room temperature in the jar. It must not be in the sun—this is important in this part.

On the third day, your sourdough needs to be fed, and it's quickly done: Add 2 tablespoons of flour and 2 tablespoons of cold water to your sourdough and mix it all well again so there are no lumps in the dough.

Now let your sourdough sit and do the work for you.

Look at and smell your sourdough on the 4th-5th day. When it starts to smell sour, it is getting ready for use. If you are brave, you can also taste the sourdough: It should be sour and taste a bit like beer.

From now on, you need to keep your sourdough alive.

If you are not going to use the sourdough immediately, put the sourdough in the fridge with a loose lid. The sourdough can keep for about a week in the fridge without being fed.

When the sourdough is to be used, take it out of the fridge and feed it. Let the sourdough wake up - this takes anywhere from 10-24 hours at room temperature.

When the sourdough is ready, take the desired amount of sourdough, and refresh the remaining sourdough: Mix 1/2 deciliter of wheat flour, 1/2 deciliter of rye flour, and 1 deciliter of cold water into the jar with the sourdough. There should be no lumps.

Put the sourdough back in the fridge with a loose lid until you need to bake again.

I've made a small list with some good advice for keeping the sourdough going:a) If your sourdough separates, so a brown liquid forms on top, just stir your sourdough well a few times a day. It will come together again.b) If a crust forms on the sourdough, remove and discard the crust. Stir the sourdough through and let it sit again with a loose lid.