It is important for you to use the right ratio of culture to milk. The right amount of milk will give a nutritious environment to the bacteria to allow it to culture and thicken properly.
To make the culture starter, you need to make sure that the temperature of the milk is kept constant and that the yogurt is not disturbed in any way. For thermophilic starters, you will need to make sure that the temperatures stay around 105 to 112°F and for mesophilic starters, the temperatures need to stay around 70 to 77°F. For the former version, you will need special appliances to control the temperatures, but the latter can be made on your countertop.