Divide the sweetened condensed milk among the 3 or 4 jars. (Make sure the jars you choose will fit in your crockpot with 1-2 inches of water covering them.) Seal the jars with clean rings and lids. Place in your slow cooker, making sure they do not touch.
Fill the crockpot with water to cover 1-2 inches above the tops of the jars. Place the lid on the slow cooker and cook on low for 10 hours.
When they’re finished cooking, remove the jars carefully with tongs (they and the water will be HOT!) and place them on your kitchen counter to cool to room temperature.
Once cool, refrigerate until using. Will keep for up to 4 weeks in the refrigerator.
Some of the rings may get little rust spots from being in the water all day. Just wipe the rust off with a kitchen towel.