To go with Sathya's Sambar:
I have never made idlies from scratch and therefore don't have a 'from scratch' recipe to share. But, I have used the boxed packages that you can easily get from any local Indian/South Asian grocery store. One of the most common brands is Gits. You will have to get an Idli Maker, but that too can be found at a local India/South Asian store. You just put it inside any large pot you have. Idlies are steamed to cook.
The most important thing when mixing up the contents of the box with the water is to NOT over stir as this can cause the idlies to be more dense/flat. But the trick is to stir enough so you don't have too many big clumps of doughy idly mix.
Serve Idlies with hot sambar (see Sathya's Sambar recipe) or just eat with melted butter, non-hydrogenated butter substitute (I like Earth Balance) or the 'sinful' but delightful ghee!