In a grinder:
When using a grinder soak the rice, methi and urad dal together. Soak for 6 hours. While grinding keep adding 1 table spoon water at regular intervals – about once in every 5 minutes. This is very crucial to get the best batter. You know the batter has turned out correct when its very fine and the consistency of the final batter is like cake batter ( for those who prepare cakes).
To be exact I would suggest that it is better to measure the amount of water also. If you soak 4 cups boiled rice (idli rice) and 3/4 cup urad dal and drain water completely before grinding, then you can add 2 and 1/2 cups water. First add 1 cup and the remaining keep adding as the batter absorbs.
This will definitely give you the right consistency. Mix with salt and leave for fermenting.
When you prepare the idlis, take out the required amount of batter in another container and mix with 1 or 2 table spoons water and make the idlies. This way the left over batter will remain fresh for more than a week.
some of the following tips to prepare idli was given in links below. surely you will get informative result.