Dry roast semolina in a non stick pan on low heat for 5-7 minutes.
Transfer this roasted semolina in a bowl and let it cool completely.
Add curd and salt and mix well.
Keep aside for 15-20 minutes.
Add water and mix very well.
Add baking soda and whisk well for a minute. But make sure before adding baking soda, the moulds should be greased and steamer be ready with boiling water.
Pour batter in each mould upto ¾.
Put the moulds in steamer and steam for 15 minutes.
If you don't have the idli mould, take any small bowls/katori, grease it with oil and pour just 3-4 tbsp of batter and steam for the same time.
Check the idlis by inserting a knife or toothpick and if knife/toothpick comes out clean, means idlis are perfectly done.
Take the moulds out and let idlis turn slightly cool and then invert.