In your menu items, make sure "Show Author" is set to "Show".
Now assuming you have enabled the plugins, each user will be able to log into your site, and go into their profile to see the new author link field.
Click on the Edit Profile button
Now you can add in the link to your Google+ page.
And all the written by section of your articles will be turned into links to your Google+ profile, and it will automatically append the ?rel=author