Creating an enfold-child child theme

I want to create a child theme for my enfold.

I followed the standard instructions from wordpress about creating child themes.

Where do i have to put my css?

1) leae it in custom.css in parent theme?
2) Put it in style.css in child theme
3) or put it in custom .css in child theme?

Also when i create a file in child theme do i have to follow the directory structure? I mean do i have to put the custom.css in css/custom.css in child?