Go to the Templates page and find the link at the bottom: Clone Boomla website by its domain. If you can not find it, hit Ctrl+F and use your browser's search, it's there just de-emphasized. Alternatively, here it is.. :)
Enter the website's domain that you want to clone, for example
On the next page, enter the new website domain you want to use, for example
If you are using a custom domain, you may want to point your domain to the Boomla infrastructure if you haven't already.