Prepare the green chilies and transfer into a small bowl. Bring the water to boil and transfer it to the small bowl. Let the chilies sit in the boiled water for about 10 seconds. Drain and discard the hot water.
In the same bowl, add in the rice vinegar, salt and sugar. Stir to blend well. Transfer everything into a small glass container or canister jar and leave it overnight in the fridge. The pickled green chilies will be ready overnight.