The bacteria that ferment milk into yogurt are typically (there are exceptions) thermophilic bacteria, active at elevated temperatures. Therefore, to make nice thick yogurt you must incubate it, maintaining it in a temperature range between 110° and 115°F/43° and 46°C.
Use a good quality milk. Full cream milk yields a luscious and thick curd.
Always boil the milk before making the curd. This ensures that the milk does not spoil during the fermentation process.
When you make yoghurt in clay pots, the clay absorbs the excess water and so the yoghurt is much thicker and richer
The milk should not be hot. If it is very hot the milk may coagulate, and you might end up getting a grainy curd.
If its little hot, then the curd does not become thick and is a bit runny with some whey in the curd.
If the milk is cold, then the curd will not be set at all.
The milk must be warm. Just dip your little finger in the milk and you should feel warm not hot.
It is very important to dissolve the curd culture uniformly in the milk. Whisk very well with a spoon or a whisker.
If you stay in a cold place or have cold temperatures, then cover the bowl or pan in which the curd mixture is with a warm towel or warm blanket and keep it in a warm place in your kitchen.
In summers, the curd will set faster than in the winters. So, remember this point.
You could use any quantity of milk you want to make the thick curd.
For half a litre milk, 1 to 2 teaspoons of curd culture work fine. Increase proportionally for larger quantities of milk.
Once the curd is set, refrigerate the curd.