Soak rice and urad dal + methi seeds for 6 to 8 hours.
Wash and grind urad dal+ methi seeds till smooth. It should double up the original quantity. (depends on the quality of urad dal). Do not add more water. Just sprinkle when needed.
Take it in a bowl. Then wash rice, add poha and grind till smooth with required water.
Take it in the same bowl with ground urad dal.
Add salt. Ferment overnight.
Check for consistency. Add water only needed.
Video
Notes
Add poha to rice only before grinding.
Instead poha , you can add cooked rice.
Batter will be good in first day to make idli .
The following days will be good for making dosa, paniyaram or uthapam.
For dosa the batter should be in flowing consistency. For idli, should be thicker than that.
I grind in wet grinder. I haven't used mixer or blender to make batter, so I'm not sure about the results you get from it.