Spread out the cheesecloth or dish towel in the colander or strainer, and put the colander over the bowl.
Pour the milk into the pot and add the salt. Put the pot on the stove and turn the heat to medium.
Bring the milk to a full, rolling boil, stirring it occasionally to make sure the milk isn’t burning on the bottom of the pot, 15 to 30 minutes, depending upon your stove.
Add the lemon juice and/or vinegar, then turn the heat down to low and cook, stirring constantly, until the mixture curdles, 1 to 2 minutes. The mixture will separate into curds (the thick, cottage cheese-like part) and whey (the liquidy part).
Turn the heat off and let the mixture sit at least 20 minutes and up to 2 hours.
Using the slotted spoon, carefully remove the curds from the pot and put them in the colander. Pour the remaining liquid into the colander. Let the curds drain for 15 to 20 minutes (The longer you let the mixture drain, the thicker the ricotta will be). Set the whey that collects in the bowl aside (you can keep it [see right] or throw it away).
Eat or use the ricotta right away, or cover and refrigerate up to 2 days.