To begin, pour the milk into a canning jar and then place the jar in a deep pot. A Dutch oven works perfectly.
Pour in enough water, that the water level is at least as high as the top of the milk in the jar. Add in the vinegar to the water. Now bring the water to a boil and let it go. Have an additional tea kettle or other pot of water simmering along side it.
Every 20 minutes or so, give the milk a stir. I just use a butter knife to do that. As the water boils, you will have some evaporation, so add some of the hot water from your tea kettle to the boiling water in the pot to keep the water level at the top of the milk. The reason I have you add the vinegar to the water, is that as you boil it, you can get mineral deposits from your water deposited on the jar. This prevents that from happening. It’s a canning trick!
You are going to let this boil, stirring and adding water as needed for 2 1/2 hours.
Allow it to cool and use it any way you like.
Store in an air tight container in your refrigerator.