To make your own sourdough culture, mix equal parts flour and water in a sealable jar - 75g of each works well. The mixing action traps natural airborne yeast particles in the flour and water mix, and they begin to feed on the flour in the jar creating a living yeast colony. Leave the jar somewhere warm and remember to feed your sourdough equal parts flour and water every day. By 5 days in it should be bubbling and ready to use to bake our sourdough bread.
Feed your starter 12 hours before baking with 75g flour and 75g water, leave out of the fridge for 12 hours before baking to enliven. Put the flour into a bowl and add 300g of your sourdough culture. Then add the warm water and the salt. Mix it all together and then turn it out onto a table for kneading. It is a wet dough so you have to work it for a long time (15 minutes by hand) before it will start to come away from the table. The finished dough should be very soft but doesn't stick to the back of your dry hand. If it does, work in a little more flour.
Feed your starter for the next time with 75g flour and 75g water and store back in the fridge.
Once it's stretchy, put it into a bowl and leave it somewhere warm to rise for 2 hours.
Shape the dough on a lightly floured surface to fit a proving basket or loaf tin. To do this, stretch the dough out into a long rectangle then fold each outer third inwards. Knuckle down a seam at the bottom of the dough nearest to you, then roll the dough down from top creating a tight loaf shape.
Dust the proving basket or loaf tin with flour and roll the dough in flour to stop it sticking. Put it into the basket, cover it and leave it to rise in a warm place for a second time, for about 8 - 12 hours.
To bake it you will need a baking stone or a heavy metal baking tray. Get your oven very hot at 240°C/gas mark 9 and heat the stone or tray. Carefully turn the loaf out onto the hot stone or tray - be careful not to knock any air out. Give it your signature cut on the top then put the dough in the oven and throw a cup of water into the bottom of the oven to create steam. Bake for 30 minutes until golden.