To create your own sourdough starter, you'll need a kilner jar. Mix half a cup of flour with half a cup of water in the jar and leave at room temperature, uncovered, for 24 hours. Add another quarter cup each of flour and water, stir, and cover with the lid. Repeat daily for two to three weeks. You'll need to tip half the mixture away at various stages otherwise you'll have way too much. Over time, the yeast culture will begin to develop and strengthen, and by the end, your concoction will be strong enough to make bread. After that, you can keep the starter in the fridge and just feed it every time you use some to make bread.
I used organic rye flour to kick mine off, switching to wheat after a couple of months. At the beginning, you're best using mineral water rather than tap water, because the chlorine risks killing the yeast. Once your starter is established, you can switch to tap.
I'll devote a subsequent post to using the starter to make bread.