Starter: 100ml/3-4fl oz of water mixed with 100g/4oz of flour (a little of it wholemeal). It would be wise to use spring water. Allow paste to sit at room temperature. Cover the bowl by placing it in a plastic bag. After two or three days, your wild yeast should be showing itself with fine bubbles in the paste. It is now called the leaven.
In order to build up your colony of yeast cells and to keep it lively, you must feed it regularly, every second or third day. Fresh flour is its food.
During these early refreshments use quantities in thirds: one third leaven to one third fresh flour, one third water, and it will continue to keep as a wet paste. The 200g of starter will increase to 600g of leaven. A couple of days later, the 600g will rise to 1.8kg. Before each refreshment, throw or give away half of the leaven to avoid stockpiling an unnecessary quantity. After several of these refreshments it will be time to turn the wet paste into a firmer, more dough-like leaven. It should have a sweetish and pleasant smell in its sourness. If it is sharp and acrid it will have been invaded by unwanted bacteria, an accident which is not your fault. Discard it and start again. The first 200g yields 600g and, assuming that you have been throwing away half of the leaven before each refreshment, then the 600g divided in half should yield 900g (300g x 3). This time, the 300g of leaven will cease being a wet paste, and will become leaven suitable for the bread.
Mix 150g of leaven (throw away the rest), 150ml water and 250g flour (quarter wholemeal) into a dough, adding a little more of either flour or water in order to get the soft consistency of dough. Knead to develop gluten. Cover and leave in a warm place. After three to six hours, it should have matured sufficiently. Take 350g of this leaven, and keep aside the remaining 200g which will seed bread for an age to come. (In two or three days time it will need refreshing.)
For sourdough bread: mix 350g leaven, 5 to 10g salt and 250ml warm water into a dough, making any adjustments of either flour or water that will give a firm and pliable dough. Knead until the gluten is well-developed and dough feels strong and stretchy. Place in a tin proving basket, or improvised proving basket made by putting a floured cotton cloth in a round bowl. Leave it to prove for several hours, well covered, in a warm part of the kitchen. After three to five hours, it should have proved well enough to be baked in a very hot oven. The baking time is 40 to 60 minutes and the bread should have a thick crust with chewy and open-textured insides.
If you are going to use the leaven daily, keep it at room temperature. Refreshments should be made every two or three days to maintain a vigorous and healthy culture. If you want to bake only once a week or infrequently, it is common to keep the leaven in a crock in the fridge. Bring it out early enough for it to achieve room temperature before refreshing it for bread-making. If it has been left dormant in the fridge for long periods, take it out and let it sit at room temperature for at least a day, and give it one or two refreshments before returning it to the fridge.