It’s a piece of cake. All you need is patience. If you use skim milk you’ll get skim-milk quark; if you use whole milk you’ll get whole-milk quark. Isn’t life simple?
In a large saucepan, combine 6½ cups (1½ l) milk and 3 cups plus 2 tbsp (750 ml) buttermilk and warm over medium-low heat to 100°F (40°C). I check it with my meat thermometer, but if you don’t have one then heat the milk until it’s the temperature of hot bathwater and you see the milk begin to thicken.
Remove the pan from the heat, cover it, and wrap it snugly in a towel. Place the pan in a warm spot for 24 hours, until the milk is thick.
Pour the milk into a sieve lined with a clean lint-free towel or several layers of rinsed and squeezed cheesecloth and set the sieve over a bowl. Let it drain for a few hours.
Now you have at least 2 cups (500 g) quark. Keep the quark in clean jars in the fridge.