Here are 2 methods for mixing and heating the milk. After that, the process is the same.
Heat in a heavy 3 liter pot:
1 liter of water
When bubbles start to form, turn off the fire.
½ blender of water
1 C whole milk powder
Pour into pot with hot water.
Repeat until pan is nearly full, using a total of 3½ C whole milk powder.
Heat in a heavy pot:
3 liters whole milk
½ C whole milk powder
Stir gently with a whisk to mix. While you're stirring, put a very clean finger in the milk to judge the temperature, and count to 10. If your finger starts to tingle as you're nearing 10, the milk is hot enough. If your milk somehow gets too hot, just wait for it to cool a bit and test the temperature again.
(When using Method B, be careful about the milk getting browned on the bottom of the pan. Method A is easier in this regard, as heating the water nearly always sufficiently heats the milk.)
When milk is the right temperature, add and stir in slowly and gently with a whisk:
about ¾ C whole plain yogurt
Put the lid on the pan. Wrap the pan in 2 big towels and leave in a safe, if possible, warm, place for 8 hours. At the end of 8 hours, it should be lovely yogurt. Unwrap and put the pan in the fridge. Let it rest for another 8 hours or so. Serve. It will keep for a week or two in the fridge.
To make a simple Greek-style yogurt, simply strain yogurt in a coffee filter, cheesecloth, or thin, clean handkerchief for a few hours. This can be eaten plain, or mixed with grated garlic, minced onions (sautéd or raw), cumin, salt, pepper, or whatever you like, and served as a dip for chips and raw vegetables, or as a condiment for Mexican food.