using child theme

Hi, I would like to use a child theme so that my customizations wont be overriden later, but can't figure out how exactly to do so looking at your file structure. I am familiar with basics on setting up child themes though. Am I supposed to just edit the files in the _inc folder? or the themes/blogs-mu-child/_inc?

And how about if I want to make changes to other template files like header.php?

thanks!