Place the lentils in a large saucepan, cover with plenty of water and bring to a boil (generally, lentils should be cooked at a 1:3 ratio, lentils to water). Simmer over medium heat for 30 minutes. Drain, then transfer to a large serving bowl and allow to cool to room temperature.
TIP: I recommend tasting the lentils after about 25 minutes to see if they are the consistency you want — more firm (al dente) is best for salads.
Plan ahead: You can prepare the lentils up to 2 days in advance. Let cool; cover and store in the refrigerator.