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 example.boomla.net.
On the next page, enter the new website domain you want to use, for example foo.boomla.net or example.com.