Pour your milk into a small saucepan and heat it over medium heat until it reaches about 180°F you'll see small bubbles forming around the edges and steam rising. This step isn't strictly required if you're using pasteurized milk, but it's worth doing. Heating helps reorganize the milk proteins so the final product thickens more reliably, and it also eliminates any competing microbes that might affect flavor or texture.

Remove the pan from heat and let it cool down completely to room temperature — around 70–77°F. This is important. If the milk is too warm when you add the starter, it can kill the bacteria. You can speed this up by setting the pan in a bowl of cold water.

Pour the cooled milk into your clean quart jar and stir in ¼ cup of your filmjölk starter. Stir gently to combine.

Put the lid on your jar and set it somewhere at room temperature between 70–77°F is ideal. A kitchen counter usually works great in most seasons. Leave it undisturbed for 18–24 hours.

How do you know when it's done? Tilt the jar gently. When the filmjölk pulls cleanly away from the sides of the jar as one cohesive mass, it's ready. It should look slightly thickened and custard-like, not watery.

Transfer the jar to the refrigerator. It will continue to thicken slightly as it chills. Filmjölk keeps well in the fridge for up to 2 weeks.