Heat your leftover whey from making mozzarella or hard cheese to 200 degrees. Use only fresh whey, no more than 3 hours old. You will start to see tiny white particles (albumin protein) floating in the whey. You can add a little vinegar if it is not precipitating well, about 1/4 Cup per 2 gallons whey.
Line a colander fine cheesecloth (butter muslin, not regular cheesecloth), place over a large pot, and pour whey through. Tie ends of cheesecloth and hang for a couple hours.
When drained, place ricotta in bowl and add salt to taste. Add a bit of cream for a richer, more moist cheese. You may also flavor with different herbs if desired.
Whey from different cheeses will yield different flavored Ricotta. It will keep a couple weeks in the refrigerator. Yield: About 1/2 pound Ricotta per gallon of whey.