Give it a title; select Personal Address Book for the template.
When done, File->Preferences->User Preferences->Mail->General
Add it to the list of local address books.
Or, you could just make a copy of an existing address book at the operating system/file level and then do the file->preferences bit to add it to the list of address books. If you do it this way, you’ll have the same entries in both – and probably want to delete them from one of them.