So ...

Upon activation, go to Settings => WP Favorite Posts to access the plugin setting configuration page:

The very first option on the settings page is pretty important. You can enable the "Add to favorite" option for registered users only, or you can leave this checkbox unchecked in order to allow all visitors to see and use the "Add to favorite" button.

Next, you can choose where to show the "Add to favorite" link (before or after post).

Then you can select the image icon for your "Add to favorite" link. Optionally, you can also upload your own image (or don’t show any image at all).

Next, you can define the number of posts you would like to show on your favorite posts page.

Finally, you can enable or disable plugin statistics. I'd recommend you to keep enabled otherwise, you won't be able to display your most favorited posts in the sidebar.

Once you are happy with your choices, don't forget to click the "Update options" button!

And below you'll see the "Label Settings" panel that will allow you to customize the labels (text) for various links:

And that's it ... You can now visit your posts to see the plugin in action. Here's the live result in my case:

OK. Now let's see some closing thoughts ...