This tool allows you to request a site cloning between two of your sites. The most import thing to be careful of is being aware to go to the destination site (the site where all the contents will be overwritten).
The you will be presented a list of source sites which you belong to admin egroup. Those are the source sites from which you can perform the clone to the current site. Simply click on the 'Clone from' button of the site you want to clone from. A confirmation message will appear. Be careful, after you confirm it, there is no way back, so it is recommended to perform a backup of the destination site before cloning it if you are not sure.
In case you have been recently added to an admin egroup of a site, and you can't see the site in the sources list, some additional time might be needed by the egroups application to propagate the changes.