Solution:1
You can check out the code of plugins that already do this task. Here is a link of some plugins that clone a WordPress website. You can download them and study the code.
Solution:2
MultiSite Clone Duplicator adds a “Duplicate Site” functionality to your network installation.
It allows you to clone any site of your network into a new one : all data, files, users and roles can be copied.
It is useful when you want to create multiple sites from the same template : Don’t waste your time copying the same configuration again and again !
Simple and user-friendly, this plugin extends WordPress core network’s functionalities without polluting the dashboard.