Way of preparation:

Milk is boiled. It is left hot for 30 minutes. Then it is cooled down to 42 – 45 °С (107 – 113°F). The starter culture of one sachet or one capcule is added and stirred well until it dissolves. Inoculated milk is left to ferment at 42 – 45 °С (107 – 113°F) °С for 8 – 10 hours without stirring. If inoculation is performed in the evening, in the morning milk will coagulate (the milk structure will be thicker). It is placed in a refrigerator without stirring. After cooling at 10 °С it will be ready for consumption.
The quality of the finished product depends on the quality of the milk which is used. If the result is not satisfactory, try another milk.
Storage: Keep it in temperatures between 0 and 10 °С.

You can see how yogurt is prepared here