In a heavy pot, over direct heat, heat 2 quarts of whole milk to 200 degrees.
Add vinegar or lemon juice and bring the temperature back up to 200 degrees. You will see very tiny white particles (albumin protein) floating in the whey. Remove the pot from the heat and set it, covered, to rest undisturbed for about 15 minutes.
Line a colander with very fine cheesecloth (butter muslin, not regular cheesecloth.) Tie the ends of the cheesecloth together and hang the ricotta to drain for an hour. The longer you hang it, the drier your finished cheese will be.
When it has drained, place the ricotta in a bowl and add salt to taste. Add a little cream for richer cheese, also add some fresh herbs if desired.
This Ricotta will keep for about a week in the fridge.
Notes
This "Ricotta" will be drier and not as smooth and creamy as the traditional "real" Ricotta made from whey, but is still a good substitute in most recipes.