Soak both the daals overnight or 6-8 hours, grind it to smooth paste.
Now take the curd and beat it nicely with a spoon along with salt and 1/4 teaspoon of sugar.
Fry zeera and the red chillies without oil and grind it to powder, add this to the beaten curd with very little water.
Now fry the baras in oil, and as soon as you take it out put the baras in warm water for 5-10 second, after that take the baras out and squeeze any excess water with hand and put it in the curd.
Finally, you can garnish your dahi bara with coriander leaves. If you like you can add mustard, jeera, curry leaves and asafoetida seasoning to this.
To make this dahi vada more appealing you can garnish little tomato ketchup and date and tamarind chutney.