Mix milk and salt in a pot. Cook over medium-high heat, stirring every few minutes, until the milk reaches 185°F. If you don’t have a candy thermometer, bring the milk to a simmer but do not let it boil.
Stir in the lemon juice or apple cider vinegar, reduce heat to medium, and let sit for one minute. You may see the milk start to curdle.
Gently stir again, turn off the heat, and let sit undisturbed for ten minutes.
Line a colander with several layers of cheesecloth and place it in a bowl to catch the whey. Pour the milk mixture into the colander.
Pull opposite corners of the cheesecloth together and tie a knot. Hang the bag over a wooden spoon above the pot to catch drips. Let drain for a couple of hours.
Transfer the ricotta cheese to a glass container and refrigerate.