How to make a duplicate child theme

I’m finding that some stuff I need to do will probably require me to have a child theme so I can change some php. However, to start I’d like to have the child theme have the exact characteristics as its parent theme.

For storefront I created a simple child theme with one file, style.css into which I ‘import’ style.css from storefront.

This left me with no styling.

How do I import all styling into my child theme? I need it where my child theme is updated automatically with storefront theme updates.

Thanks!