Line a colander or a fine-mesh sieve with a large piece of lightly dampened cheesecloth that has been folded over itself a few times. Place the colander over a large bowl.
Add the milk and salt to a large heavy-bottomed pot.
Gently heat over medium-high heat, stirring frequently with a wooden spoon to prevent the milk from scorching on the bottom of the pan.
If you have a thermometer, we are aiming for a temperature between 180°F and 190°F. If you do not have a thermometer, look for little bubbles close to the edge of the pot, and the formation of a slight film. The milk will become foamy, but should never hit a rolling boil. This will take approximately 10-15 minutes. Milk can boil and scorch faster than you think so just have a little patience and watch it closely. Lower the heat to low.
Slowly add the lemon juice (or vinegar).
Gently agitate the mixture for a minute or so. You will notice the ricotta curds separating from the whey (the yellowish leftover liquid).
Remove from the heat.
Cover the pot and let stand for about 20 minutes.
Using a slotted spoon, carefully transfer the curds into the cheesecloth, allowing the whey to drain. Let it drain for 10 -60 minutes, depending on your desired ricotta consistency. For a creamy consistency don't let it drain too long. The longer it sits, the drier and thicker it will be. Transfer the homemade ricotta cheese to a bowl, season with salt to taste, and stir gently.
Use immediately or refrigerate in an airtight container for 3-4 days.