A recipe for a classic sourdough starter is found at BBC.co.uk and requires full-fat plain yogurt, skim milk, strong white flour, water and milk. It takes five days to prepare.
On the first day, begin preparations by heating 6 fluid ounces of skim milk in a saucepan over a gentle heat. Next, spoon 5 tablespoons of fresh, live, full-fat, plain yogurt into a bowl together with the warmed milk, and mix to combine. Cover this mixture, and leave in a warm area for 12 to 24 hours or until thickened. Check intermittently, ensuring that the mixture is stirred again if any liquids become separated.
On the second day, mix 4 ounces of strong white flour into the yogurt. Next, cover the mixture, and leave at room temperature for two days. The mixture will begin to form bubbles and smell sour during this time.
On the fourth day, add the flour, water and milk to the starter. Cover the mixture once again, and leave at room temperature for 12 to 24 hours.
On the fifth day, the starter should be very bubbly. Discard half of the starter, then add flour and water to the remaining half and combine thoroughly. Cover and leave at room temperature for 24 hours.
On the sixth day, the starter should be complete. It should be covered, stored in the fridge and used when needed. Feed once every five days.