Add milk, cream, and salt (if using) to a pot.
Warm milk to between 195°F and 200°F on medium heat. If you do not have a food-grade thermometer, warming will take between 30-40 minutes, depending on your burner. When you see foam and bubbles form around the edges of your pot, and foam form on the top of the milk/cream mixture, the liquid has most likely reached the proper temperature. (See video.)
Once the milk/cream mixture reaches the proper temperature, turn off the heat but leave the pot on the burner. Pour in the vinegar and stir gently. Allow the milk/cream mixture to sit for 5 minutes.
After 5 minutes, you should see curds forming. If you do not see many curds, add a bit more vinegar. No more than 2 tablespoons of additional vinegar should be necessary to obtain curds.
Once the curds have sufficiently formed, strain the curds and the liquid (whey) into a cheesecloth or flour sack towel lined colander or mesh strainer that has been placed over a bowl. The whey will drain into the bowl, and you will be left with the curds in the lined colander or strainer. Allow the curds to drain for 30 minutes for a cream ricotta or 1 hour for a firmer ricotta.
Once the ricotta is done straining, transfer to a container with a lid and store in the refrigerator. The ricotta will stay fresh refrigerated for about 2 weeks. The "acidic" whey will last approximately 6 months, refrigerated. See video for ideas on how to use the acidic whey.