Wash the dal until the water comes out clean (6-7 times). Soak it overnight in plenty of water. Next morning wash it again. Then with the help of a mixer-grinder, grind it to a smooth thick batter using very little (almost no) water. To check the consistency (should be very tight), take a little batter in your index finger and thumb; it should stay between your fingers and not drip off easily. After grind the entire dal shift the batter in a big bowl. The next step involves some workout if you do it the traditional way ;) Use four fingers (not the thumb usually) to whisk the batter.
Alternatively, you can use a wire whisk but where is the fun in that?
Whisk until the mixture becomes almost double in volume — airy and light. To check the consistency, take a bowl of water and drop a grape-sized dollop, if it floats half way in the water it means it is ready to go in the sun!
Now, take a clean cloth and spread it out in the sun. Make sure it is smooth and has no wrinkles. Take a spoonful of batter and start making small dollop using your fingers. It needs some practice. You can try using an icing cone. Finish all the mixture and let them dry in the sun for the whole day. Next day dry them again. After drying for two days slowly take out the dollops one by one and spread them in a plate and again for the next 5-6 days let them dry in the sun. Remember to not leave it out at night! Store them in a airtight container afterwards.
Note: Check the weather before making bori, you need lots of sunshine for 5-6 days in a row.