Sharing previews are yet another thing missing from WordPress which should really be included by now. It’s dead easy to install a plugin and have it all done for you, but why bother with slowing down your site with needless plugins when you can add a tiny bit of code to your functions file.

//Add Twitter Cards Meta Info
function add_twitter_card_info() {
    global $post;
    if ( !is_singular())
    echo '<meta name="twitter:card" content="summary"/>';
    echo '<meta name="twitter:url" content="' . get_permalink() . '"/>';
    echo '<meta name="twitter:title" content="' . get_the_title() . '"/>';
    echo '<meta name="twitter:description" content="' . get_the_excerpt() . '"/>';
    echo '<meta name="twitter:site" content="SumTips"/>'; //optional: username of website
    echo '<meta name="twitter:creator" content="SumTips"/>'; //optional: username of content creator
    if(!has_post_thumbnail( $post->ID )) { //use a default image if no featured image set
        $default_image=""; //replace this with a default image
        echo '<meta name="twitter:image" content="' . $default_image . '"/>';
        $thumbnail_src = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'medium' );
        echo '<meta name="twitter:image" content="' . esc_attr( $thumbnail_src[0] ) . '"/>';
    echo "\n";
add_action( 'wp_head', 'add_twitter_card_info');

Simply paste in the above code to the functions.php file in your WordPress theme and validate all has gone correctly using the Card Validator.