Soak the Moon dal in plenty of water for 4-5 hours. Drain the water. Then grind it in a mixer along with cumin seeds, salt and water as required to get the desired consistency for Dosa. Then the batter is ready to prepare Dosa.
The method to prepare the dosa is the same as for any other regular dosa.

Note

If you live in India, then you can grind the Moong Dal and Cumin dry from a local Dry Grinder Shop and store it in a air tight container.
Any time you feel like preparing Dosa, just mix it with water and salt and you are done!