Whats the best way to clone the root site of a Multisite install then?

Hi there, how do you copy over a menu from another site?
It seems only does content.

Your plugins say can't do it from the root site.

I'm wondering what steps to do this manually then?
I would like to copy/clone the root site (which was a standalone wordpress install but now converted to multisite) which includes the theme, menu, content so I can have a good start to edit the new content which is based on the original one.

What is the best steps to do this?