First of all soak lentils overnight or for at least 2 hours.
Boil it with water and add salt to it as per taste. Dal should not be very soft cooked. It is good to boil it in a pan rather than in pressure cooker. Boil it for around 10 minutes, this will make lentils soft enough to make a chat.
Drain all the water and cover it with the lid for another 5 minutes. Meantime prepare stuff for garnishing.
Heat oil/ghee in a small vessel and add cumin seeds. When they start crackling take off the pot from gas.
Serve lentil to a plate and spread this tadka (tempering) on it. Sprinkle red chili powder & chopped green chili on top of it.
To this add chopped boiled potatoes, cucumber, and onion. Now add chopped tomatoes and coriander leaves.
Finally, add lemon juice. Moong dal chat is ready to eat now :).