Wash the udad dal and fenugreek seeds. Soak in plenty of water for 2 hours.
Wash the idli rava and soak till immersed in water, at least for 30 minutes.
Drain dal well and grind in the mixer until fluffy and soft, using upto 1/2 cup of water.
Squeeze out excess water from the soaked idli rava and grind to a coarse batter. No need to add water to this.
Remove both the batters into a large vessel. Mix well with your hand. Cover and keep aside for 6-10 hours until well risen / fermented. In summers it may take just 6 hours, while in winters, you may need to keep in a warm place like the oven, with the light turned on for extra warmth. These batters harness the power of naturally occurring wild yeast to rise.
Once the batter is fermented, either refrigerate it or use it immediately to make idlis / dosas.
I prefer not to add salt before fermentation. I take as much batter as is required for each batch of idlis / dosas and add the required salt just before using it.