Summarize:

Pour 1 cup (250 mL) dried lentils out on a kitchen towel, plate, cutting board, or other clean surface. Pick through the lentils, sorting out any rocks, bits of soil, or other debris as you run across them. Also discard any lentils that appear to be damaged.  While reputable sellers do have a sorting process of their own, it is possible that their machines can miss the occasional rock or bad lentil. As such, it is recommended that you sort through the lentils by hand before making them. Note that green, brown, and French lentils work best if you plan to prepare and serve them on their own. Red, orange, and yellow lentils tend to get mushy when cooked, and are better when prepared for sauces, stews, and soups. Place the lentils in a colander and rinse them under cool water for 30 seconds or so. Turn off the water and gently shake the colander to encourage excess water to drip out.  A strainer, sifter, or other fine-mesh utensil can also be used in place of a colander. Simply make sure that the tool you use has small enough holes to prevent the lentils from falling through. Do not soak the lentils. Most dried beans and other dried legumes need to be soaked for a period of time in order to soften them and remove some of the compounds known to cause digestive upset. Lentils, however, will become mushy if allowed to soak in the same way dried beans might soak, so pre-soaking them is not recommended.
Sort through the lentils. Wash the lentils.