Take a cup of flour and a cup of water, stir them together, and wait. Once a day, tip half of it away, and add another half a cup of flour and half a cup of warm water. And after about three days, you might have something like this.
Which you may decide is nicely bubbly and frothy, and so could just be the sourdough starter you have been waiting for. So, you add a full cup of flour and another one of warm water, and pour it into a bowl.
And wait overnight, until it gets all frothy again, at which point you can take two cups of it, add flour, oil, salt and sugar, and have your very own sourdough loaf. Ideally, you keep the remaining cup as your next sourdough starter. In reality, the bowl tips upside down, spreading fermented flour and water across your kitchen floor, you mutter very rude words, and hope the loaf turns out nicely.
The four day brewed loaf tastes a little bricklike, so you decide you didn't leave it long enough. And you start again, with another cup of flour and another cup of water.
And you tip half of it away, and feed it daily with half a cup of water and half a cup of flour. And you stir in the evil smelling liquid on the top, and put it away for another day. And then suddenly after a week, you discover this
And decide THAT's the frothy effect you were aiming for last time and didn't quite get. So you grow a sponge, which takes another full day. And then you add flour, salt, sugar, olive oil, and leave it to prove. And for the next 18 hours it sits in a small round solid lump doing nothing. So you go to bed, and in the morning, 24 hours after you started kneading it, you discover it has doubled in size and actually looks as though it might be a loaf some time.
So you bake it
And, ten days after you started, you have a perfect sourdough loaf. Abingdon sourdough - I'm sure it doesn't compare with San Francisco, but it tastes pretty good to me!