How to use the child theme

Hi, Rohit.
You are doing a definitely great job.
But I have a question:
You recomment to use the child theme for stylesheet modifications.
Ok I told wordpress to use your child theme.
What which css file is now the file to deal with.
When I apply changes on the parent main.css (Alltough I am using the child theme) erverything works. But is this the solution?

Thangs for answering
Gerry