This is my lentil soup, which has a lot of possibilities for experimentation, different seasonings, etc..
Since I do this by eye, this recipe is a little vague, but it's really hard to mess up lentils unless you burn them.
With all beans, it's best to keep the flame or temperature very low. Those clay bean pots can be expensive, but are really great.
Fill a saucepan about 3/4 full of water or stock, a generous pinch of cayenne, and some salt, then bring to a boil. A bouillion cube in the water can be nice if you don't have stock. Once the water is boiling, bring your flame down low and add lentils and, if you want to, pearl barley or (washed) quinoa. For a 6 qt saucepan, use 1-2 cups of lentils, depending on how many other ingredients you have.
Digression: This recipe is for green lentils (which look brown), but works equally well with French green and red (which look orange when raw). However, if you use red lentils, cut down on your cooking time by at least half an hour, possibly more. Red lentils tend to disintegrate, and when cooked they look yellow--this is how they're supposed to look. Brown lentil soup takes 1 1/2-2 hours to cook. Vegetables need about a half hour to cook.
Back to the recipe. After the lentils and barley have simmered for about an hour, add chopped vegetables. My staples are potatoes, carrots, and peeled broccoli stems. Corn is also really good, and canned corn is easy--just add it right before the end. I've had good results with sweet potato, as well. Some people like celery.
After adding the vegetables, simmer for another half hour or so, then add crushed or chopped garlic, and onions if you like them. I usually go with 3-5 cloves of garlic and a tablespoon of onion powder. (Lentils can soak up a lot of seasoning.) Now is also the time to add your other seasonings. Combinations I've used include cajun seasoning straight from the container; curry powder; red wine vinegar; chopped raw ginger and fresh lemon juice; and blackstrap molasses with lemon juice. Whatever you choose, let it all simmer for another half hour or so. Add fresh-ground black pepper and salt to taste. If you want rice in your soup, cook it separately and mix it in when the lentils are done.
You'll know it's done when the lentils break apart easily. If they're not done, they'll crumble in your mouth and have a kind of chalky taste.
Thickness of the soup depends on the ratio of water to lentils and how long they are cooked. I've never quantified it exactly. Have fun experimenting!