Dampen a large enough piece of cheese cloth that when folded over 3 - 4 times it will fit into a colander big enough to hold your curds, approx 3 - 4 c (you can also use a very small holed colander). Place the colander inside a large bowl to collect the whey. (You can save this whey to use in other things, see notes.) If you don’t want to save the whey you can drain over a clean sink.
In a heavy saucepan, over medium heat, slowly warm the milk. Stir occasionally with a heat resistant spatula or wooden spoon scrapping the bottom. Be sure the milk does not boil or it will scorch. I use a thermometer to ensure the milk reaches 185 F. If you do not have a thermometer, bring it to a low simmer. It should take about 20 minutes.
Turn off the heat and stir in the salt (if using) then pour the lemon juice (or vinegar) over the surface evenly.
Coax out the curds by gently stirring the mixture for 1 - 2 minutes, a slotted spoon works best for this to help break up the mixture. The curds should start forming immediately as they begin separating from the whey (yellowish liquid). Pretty cool! If you do not see curds or if they look too thick add another 1T lemon juice (lemon's acidity varies).
Remove from heat and let chill for 10 - 20 minutes.
Use a slotted spoon or small strainer to ladle your ricotta curds into the cheesecloth lined colander. Fold over the ends of the cheese cloth to cover if making firm ricotta. You can also top with a plate for a little extra weight.
How loose or firm your ricotta will be depends on long you leave it to drain. The more drainage the firmer it will be. Its ultimate end use will decide what consistency you desire. For a creamy, spreadable ricotta 5 - 10 minutes; a dryer ricotta for stuffing and desserts, it can sit up to 24 hours. To make a very dry, firm ricotta salata type cheese top with a weighted plate and drain for 24 hours. If draining longer than a couple of hours place in the refrigerator.
Store in a tightly sealed container in the refrigerator. Can be frozen.