Clean and presoak the lentils for a few hours, two to three is sufficient, over night works well also. The lentils will easily drink up two cups of water.
Strain them, and dump them and all of the other ingredients into a 6 quart or larger slow cooker. (A 5 quart might work, but it would be a tight fit.)
Cook on low for 10 hours or high for 5 hours. I usually do a combination of the two settings (e.g., 6 hours on low and then 2 hours on high).
NOTE: Three medium (baseball sized) sweet potatoes, chopped, can be substituted for the sweet potato purée, but the later seems to sweeten and thicken the overall soup significantly. The same with the carrots, where mincing them does seem to impact the color, flavor, and texture a bit.
NOTE: I have yet to try red lentils, so, if some brave individual does, I've appreciate hearing about how it turns out and what adjustments are required.