Trick to lentils is to wash and drain them first which removes a lot of extra skins and such, then add to at least four times the amount of water, bring to a boil, add salt and spices and a bouillon cube if you want to. Lentils are like potatoes in that they like salt and it makes a huge difference.
Simmer with a lid tilted until the water is mostly absorbed, add cabbage, onions, carrots and red pepper if you wish. It may take up to forty minutes on low heat to really cook them soft - even though the bag will tell you 15-20 minutes. If your carrots are fresh and not canned that extra time will soften them as well. If you want to run it all through a blender and make it true soup it's good.
Point : Rice has a 1 : 2 ratio with water. Lentils have a 1 : 4 ratio (some say 3)
rule of 1 : 4 - one cup lentils to 4 cups water
wash it like rice, rinse several times
no need to soak though
If you are adding other vegetables, add a bit more water so it all has a good chance to merge flavors. And don't forget the salt and/or bouillon cube, it really makes a difference. I didn't have beef bouillon so I used salt and a little butter, pepper, celery salt and then added frozen cabbage, fresh chopped carrot and a few strips of red pepper.