Soak lentils in 2 cups of water, and rice in 4 cups of water, separately- for 4-12 hours. If using fenugreek, add them to the soaking lentils
Drain the rice and discard its soaking water, and blend together the rice + lentils + lentil water + baking soda + salt in a high powered blender. (If the blender is weaker, you may need to blend the rice and lentils separately to make sure the rice isn't left too intact and grainy.)
The blended mixture should be smooth and a little drippy (think pancake batter, not crepe batter)
Decant the batter into non-reactive vessels and allow them to ferment. (Temperature variable, 80F is good enough, 110F is optimum if using a circulator or other tool to rapidly ferment.) Allow to rise fully once, use a spoon to break the bubbles, and allow to rise a second time.
Once the second rise is done, cook immediately, or refrigerate for up to 10 days before using. (Can also be frozen indefinitely.)