To make sourdough bread, you need a sourdough starter. This is a live fermented culture, which plays the same role as yeast. It's really easy to make and you can keep it for years.
The live culture gives sourdough bread that distinct 'sour' taste. During the proving stage, it begins to break down the flour, so that by the time you eat the bread, it is much easier to digest. The fermented culture is also really good for your gut microbiome.
To make a sourdough starter from scratch:
In a small plastic pot, mix together equal parts bread flour to water. About 3 tablespoons of each will do. Put the lid on then leave overnight at room temperature.
After 24 hours, remove 2 tablespoons of the mix - into the bin or down the sink, and replace with another tablespoon of water and a tablespoon of flour. Mix and leave again. You are essentially 'feeding' the live culture which allows it to grow.
Repeat step 2 for a week. After 7 days, the starter is ready to use to make bread. To check it's ready it should have a distinct sour smell.
Transfer the pot into the fridge for storage, repeating step 2 every week to keep the culture alive. It won't die if you forget, but try not to leave it longer than a fortnight.
Once you have a starter, you can grow it by adding more flour and water and then share it with a friend!