When I made lentil soup, I just rinsed the lentils really well and then cooked them for about 90 minutes.
All other beans like black, red, pinto, chickpeas etc. I cover with water, bring to a boil and then take them off the heat, leave the top on and let them sit for one hour. Drain, rinse well and cover with clean water and bring to a boil again. Cook 3 - 4 hours, covered - stirring every hour or so.
Or you can soak them 8 - 12 hours instead. (Then cook as above)