Using a child theme

Hi. I’ve realised after the recent update that I need to use a child theme so that I don’t lose the changes I’m making to my theme. However I’m not sure how it works. I’ve installed the child theme, but need to know:

Do I make the changes that I need to make to files in the Child theme? and if so do I need to make copies from the parent theme to alter?
OR
Do my changes automatically become saved in the child theme?

Thanks.