Check through your dry lentils and remove anything that shouldn't be there (small stones, other grains, etc.). Place them in a pot with the water. Heat to boiling, then reduce heat and simmer until the liquid is absorbed and the lentils are tender, 15 to 30 minutes depending on your lentils.
Either boil the chopped potatoes in just enough water to cover them, until the potatoes are cooked but not yet mushy, which takes about 10-12 minutes, then drain them. OR put the diced potatoes on a cookie sheet, toss with some salt and oil, and bake in a 400 degree oven, turning them occasionally until the potatoes are turning golden brown. (I prefer the oven version, but if the weather is very hot I'll choose the boiled version to avoid heating up the kitchen.)
Meanwhile, if you are using onions, in a big pot such as a Dutch oven, saute the onions in some olive oil. Then add the garlic and cook for another couple of minutes. Add the greens, and cook until they decrease in volume and are heated through.
Add everything to the big pot or put it in a big mixing bowl. Stir thoroughly.