To make sourdough bread you need to use a ‘starter’ mixture that takes the place of yeast. The starter takes about 5 days to develop (or you can buy it online), but once you have it you can keep it alive and use it whenever needed. It's a bit like having a pet.
Ingredients
500g/1lb 2oz strong unbleached white bread flour, plus extra for dusting
300g/10½oz sourdough...