Create child theme from an already established/modified theme?

Hi guys, I have a client who designed his website without a child theme. He hasn’t updated his theme in a while and things are starting to break. I told him he needed a child theme, however, I am in the dark as to how to create a child theme from an already established site. I searched online and all I can find is how to create a child theme, which I already know. I don’t want to break his site, as I wasn’t the one who designed it to begin with. So my questions is, how can I create a child theme of his website? Thanks!