Aside from milk, vinegar, and salt, you'll need some cheesecloth and a cooking thermometer (both of which you can find at most supermarkets). Then it's just a matter of heating, stirring, and straining.
1 gallon whole milk
3/4 cup distilled white vinegar
1 tablespoon kosher salt
Line a footed colander with a double layer of cheesecloth and set it in a clean sink. In a large saucepan, combine the ingredients over low heat. Clip a cooking thermometer to the pot and, stirring constantly with a wooden spoon or rubber spatula, heat the liquid to 140°F, about 10 minutes. Stop stirring and let the temperature of the liquid climb to 175°F. This will take another 8 to 10 minutes, but stay close by; it's important not to let the liquid exceed 180°F, or the cheese will be overcooked and become grainy. As soon as it hits 180°F, remove it from the heat and gently ladle the separated curds out of the saucepan and into the cheesecloth-lined footed colander (you need some height for the liquid to drain away). Alternatively, tie the curds in the cheesecloth with ample string and then tie the cheesecloth package to a cabinet knob, positioning it over a bowl to catch the liquid that drains from it. Allow the curds to drain for about an hour, until the ricotta is soft and spreadable but no longer wet. (The longer it drains, the firmer the cheese becomes.) Transfer the ricotta to a bowl, cover, and refrigerate if not using immediately.
MAKE AHEAD: The ricotta will keep for a week, covered and refrigerated.