Well there's many family recipes no doubt, but when I make it, it's usually:- (not exact quantities !)
red split lentils (1 small to medium bag - according to how thick you want it)
carrots (2 or 3 large, grated)
white onion (1 large, finely diced)
6 - 8 slices smoked back bacon (or a smallish smoked bacon piece/joint/gammon) or else Knorr* bacon stock cubes.
2 or more litres of water (can be Knorr* bacon stock).
Basically lump it all together and boil, remove any scum and simmer for an hour or two, covered. If it's too thick and needs more water, add some, if it's too thin, add more lentils. You can take out the bacon pieces and use elsewhere, keep some in the soup, chop and put some in the soup, give it to the dog etc.
It does itself really.
* Do I get an advertising fee for this ?