I wanted users to be able to re-use an existing SharePoint "For a selected item" workflow on other libraries without having to create one from the out of the box templates and configure it each time.
To do this, start with a document library or list, select an item and on the Flow menu, Create a Flow. Choose a template to start with and then in Flow configure what your template will be; in my case it also requested an email for reviewers and sent approval email to them.
Test the workflow is running before you do the next steps.
TO re-use the workflow, simply open the existing workflow in Flow and under the More option, select Save As, you can then save a new name relating to a different document library. Next, change the trigger site name to the new site if needed and in the List Name, enter either the ID of the list or its name (Note not url name), so for Shared Documents, enter "Documents". Update and save the Flow and then go to the new document library, select an item and then under Flow, you should see the Flow you created from your base template.