No configuration needed – just install it and activate it.
Yeah, I could have just pasted the code into my theme template, but I like to keep functional changes separate in case I ever decide to switch to another theme.
At present it will only show on individual posts. It won’t show on pages, or the front of the site, or any sort of archive page.
You can see it in action on Speed Force.
[Update:] The service no longer exists (the site is a blog now), so there’s no need for the plugin anymore.