Pick a button style and copy the code from the Embed Button page.
Or you can copy the default button style here:
*Note that you can only include the button on a Wordpress.org blog, not a Wordpress.com blog.
The code can be installed anywhere in the post loop. I'm going to install it in single.php, which is the code for the single Wordpress post page, but it can just as easily be installed in several of the other theme pages.
Save it, and you're done! Open up a post on your Wordpress blog and you'll see it right there.
Click the button, you'll see a beautiful Card for your post. The example is for this post.