1. Navigate to the WordPress plugin directory of your development installation.
2. Create a new directory called ch2-twitter-embed.
3. Navigate to this directory and create a new text file called ch2-twitter-embed.php.
4. Open the new file in a code editor and add an appropriate header at the top of the plugin file, naming the plugin Chapter 2 - Twitter Embed.
5. Add the following line of code to declare a new shortcode and specify the name of the function that should be called when the shortcode is found in posts or pages:

add_shortcode( 'twitterfeed', 'ch2te_twitter_embed_shortcode' );

6. Add the following code section to provide an implementation for the ch2te_twitter_embed_shortcode function:

function ch2te_twitter_embed_shortcode( $atts ) {
    extract( shortcode_atts( array(
        'user_name' => 'ylefebvre'
    ), $atts ) );

    if ( !empty( $user_name ) ) {
        $output = '<a class="twitter-timeline" href="'; 
        $output .= esc_url( 'https://twitter.com/' . $user_name );
        $output .= '">Tweets by ' . esc_html( $user_name );
        $output .= '</a><script async ';
        $output .= 'src="//platform.twitter.com/widgets.js"';
        $output .= ' charset="utf-8"></script>';
    } else {
        $output = '';
    }
    return $output;
}

7. Save and close the plugin file.
8. Log in to the administration page of your development WordPress installation.
9. Click on Plugins in the left-hand navigation menu.
10. Activate your new plugin.
11. Create a new page and use the shortcode [twitterfeed user_name='WordPress'] in the page editor, where WordPress is the Twitter username of the feed to display:

12. Publish and view the page to see that the shortcode has been replaced by an embedded Twitter feed on your site.
13. Edit the page and remove the user_name parameter and its associated value, only leaving the core [twitterfeed] shortcode in the post, then Update to save changes.
14. Refresh the page and see that the feed is still being displayed, but now shows tweets from another account.