I’m starting a new sourdough culture from scratch. All you need is flour, water a nice and warm spot and some time.

Recipe

50g wholemeal flour
50g universal flour
50-100g water

Mix everything nicely together and put to rest for a day. The bacteria prefer temperature between 20 and 25 degrees. Low temperature encourage lactate-bacteria growth (sour taste), higher temperature yeast bacteria. In the end you want to have both, so keeping the temperature in this range is recommended.

Also making sourdough and bread from it is one of the crucial skills everyone should know for the unavoidable upcoming zombie apocalypse :-)