Use 1heaped tablespoon of fresh live yogurt to 1pint/600ml of milk (for creamy yogurt, use full-fat live yogurt and full-fat milk; for thick yogurt, add 1-2tablespoons of powdered milk to the milk boiling in the pan).
Bring the milk to the boil in a large pan. As it bubbles and froths, turn the heat down and leave it to simmer for 2-3 minutes. Turn off the heat and leave the milk to cool to a temperature that is bearable to your finger dipped in it for a count of 10. Beat the yogurt in a bowl and strain a little of the milk into it. Beat well and gradually strain the rest of the milk into the yogurt, beating all the time. Cover the bowl with cling film or a plate, wrap it in a blanket or thick towel and put it in a warm place for at least 6 hours (or overnight). Once it has fermented and thickened a little, put it into the fridge for a few hours to set.