First of all. soak the lentils for 3 to 4 hours. If the weather is cool, then soak it little longer, may be up to 6 hours. When the lentils are properly soaked, they will not only swell but will easily break into 2 when pressed between the fingers.

Now, drain off the excess water and grind these lentils into a fine paste. Add very little water, if required, while grinding.

Beat the batter for 3 to 4 minutes either with hands or an electric beater or whisk. The batter will become fluffy which in turn will lead to soft Vadas. Do not beat too much, if the Vadas are too soft then they dissolve/break in the water and hence are not good.

Heat oil in a pan. When the oil is very hot, lower the flame and fry Vadas in it.

Fry all the Vadas and keep them aside.

Take enough water in a wide pan and soak Vadas in it.

After about 15 minutes, remove the Vadas by squeezing with both the hands. You will be surprised to see the quantity of oil floating on the water.

If possible, keep the jar in the sunlight otherwise at any other warm spot in your kitchen. Stir it daily with light hands taking care that vadas do not break.

After about 4 to 5 days, mustard seeds will swell and the water will taste tangy too. 

To serve, put one or two Vadas in a glass/bowl and fill it with kanji (fermented water). 

You can add little green coriander chutney for making it spicy.

Once ready, it remains good up to a week only thereafter the taste starts deteriorating. To increase its shelf life you can keep it in the refrigerator After it is ready.