In a bowl, measure and add in all the lentils/dal along with fenugreek seeds. Wash them all well thrice or four times. Soak in clean water (1-2 inches above the level of the lentils) for 5-6 hours.
In another bowl, measure and add in sona masuri rice and idli rice together. Wash well thrice or four times. Soak in clean water (1-2 inches above the level of the lentils) for 5-6 hours.
Just before grinding the previously soaked ingredients, wash and soak flattened rice for 10 minutes.
After 6 hours, discard the water from both of these bowls.
Next, grind the contents in the urad dal bowl first, adding fresh water as required until they are completely smooth, fluffy and light. Do not add a lot of water in one go, add in steps as required. Once ground well, transfer this to a container.
Note: My grinder required about 2 cups water to grind these. Mixer will require less.
Next, adding just enough fresh water, grind raw rice, idli rice/dosa rice and the flattened rice (poha) together until they are ground smooth. Remember the rice should not be coarse as it will not be enjoyable while eating dosa. Likewise, do not grind for too long as it will heat up the batter.
Note: I used about 3/4-1 cup water here.
Transfer this to the same container in which you have poured the ground urad dal. With your clean hand or a ladle, mix everything very well. Using your hand helps to bring in the good bacteria needed for fermentation.
Once it is mixed well, just close with a lid leaving a little gap for the air to go in. Let this ferment for 6 to 12 hours or more, depending on the room temperature.