What you need
2. Live yoghurt cultures (100 ml plain yogurt or 1 pack of powdered culture per liter milk)
3. Thermometer that can measure to at least 50*C
4. Clean stainless steel pot with a heavy "sandwich" bottom
6. Oven or other source of heat that can keep the yogurt warm for several hours
7. Yogurt containers (glass or food quality plastic)
You need live yogurt culture that can multiply in the milk and turn the milk into yogurt by turning milk sugar (lactose) into lactic acid. The lactic acid makes milk products more easible digestible, which is important as the stomach of adults doesn't provide for milk digestion any more (unlike the stomach of an infant). This live culture can be bought as a powder or in form of a live yogurt. I have tried the Togg's plain goatmilk yogurt and it works.
When making yogurt you have to be very hygienic to avoid infection of the milk with foreign bacteria. Always wash your hands and equipment thoroughly. Equipment should always be rinsed with cold water first and then washed with hot water afterwards. Equipment can also be desinfected with a little JIK in the water. Avoid contact of the milk with strong smelling liquids or smoke as that will change the taste of the milk. The pot should have a thick bottom as milk will burn very easily in thin pots/sufurias. If milk burns the protein in the milk crumbles and the texture of the milk becomes unpleasant. It will also take on a burnt taste.
After the yogurt is cultured (turned sour) it will still be quite liquid. It needs to set first. For this it needs to be cooled for several hours, best overnight. This will also stop the culturing process.
I boil my milk to kill any unwanted bacteria as it comes from some neighbours cow in various containers and I can't be sure it isn't "a little bit dirty". If the milk comes from your own cow and you use it immediately and you are also sure about your hygiene standards, it is also fine to make raw milk joghurt.
Use a thermometer to ensure you introduce the culture at the right temperature. If you are using yogurt instead of powdered culture add it to the milk at a slightly higher temperature as it will cool the milk down.
Use a whisk to stir in the powdered culture or the plain live yogurt.
The yogurt sets better if left to set in small containers than in one big bowl. I use small food-quality plastic containers with a screw lid. But you can use even used and washed yogurt pots or drinking glasses.
The containers with the yoghurt should have a lid to avoid infection with bacteria and smells. The containers are then placed in a shallow dish holding warm water. It should be just enough water to reach up to where the yogurt is filed in the containers. This will help to keep the temperature stable and avoid burning.
Leave the yogurt in the oven for 1 - 4 hours at about 50*C until it is well cultured and has reached the taste you like. It will still be a bit too liquid at this state. It only sets properly after a few hours in the refrigerator.
If you don't have an oven wrap the yogurt container in some thick towles or keep it inside a styroform container or even a small cooler box (add warm water). This will maintain the temperature for long enough to culture the yogurt.
For the yogurt to properly set (get thick and stiff) it needs to be cooled. This will also stop the culturing process so that the yogurt does not continue to get more and more sour. For this a refrigerator is needed.
To add flavour you can stir in vanilla sugar (to make vanilla yogurt) or any fruit concentrate, like the passionfruit concentrate (check out my older posts). You can also use a tablespoon of your favourite jam or even honey. I personally just eat the plain yogurt with some fresh fruits.