===================================================
========== FOR DEVELOPER REFERENCE ONLY ===========
===================================================
Elements
enable/disable description tab
enable/disable reviews tab

Styling
Primary Buttons
    Add To Cart
Secondary Buttons
    Apply Coupon
    Update Cart

Quantity Input Field

PRO - Table (Cart Page and Checkout Page)
    Table Border Style and Color
    Header Text Color
    Header Background Color
    Font Size
    Font Weight
    Text Alignment

Input & Drop Down Field 
    Border
    Padding
    Color
    Etc.

add_action( 'woocommerce_checkout_after_customer_details' ,'kia_extra_checkout_fields' );


/*********************** Backend **********************/
// save the extra field when checkout is processed
function kia_save_extra_checkout_fields( $order_id, $posted ){
    // don't forget appropriate sanitization if you are using a different field type
    if( isset( $posted['some_field'] ) ) {
        update_post_meta( $order_id, '_some_field', sanitize_text_field( $posted['some_field'] ) );
    }
    if( isset( $posted['another_field'] ) && in_array( $posted['another_field'], array( 'a', 'b', 'c' ) ) ) {
        update_post_meta( $order_id, '_another_field', $posted['another_field'] );
    }
}
add_action( 'woocommerce_checkout_update_order_meta', 'kia_save_extra_checkout_fields', 10, 2 );

// display the extra data on order recieved page and my-account order review
function kia_display_order_data( $order_id ){  ?>
    <h2><?php _e( 'Additional Info' ); ?></h2>
    <table class="shop_table shop_table_responsive additional_info">
        <tbody>
            <tr>
                <th><?php _e( 'Some Field:' ); ?></th>
                <td><?php echo get_post_meta( $order_id, '_some_field', true ); ?></td>
            </tr>
            <tr>
                <th><?php _e( 'Another Field:' ); ?></th>
                <td><?php echo get_post_meta( $order_id, '_another_field', true ); ?></td>
            </tr>
        </tbody>
    </table>
<?php }
add_action( 'woocommerce_thankyou', 'kia_display_order_data', 20 );
add_action( 'woocommerce_view_order', 'kia_display_order_data', 20 );


// display the extra data in the order admin panel
function kia_display_order_data_in_admin( $order ){  ?>
    <div class="order_data_column">
        <h4><?php _e( 'Extra Details', 'woocommerce' ); ?></h4>
        <?php 
            echo '<p><strong>' . __( 'Some field' ) . ':</strong>' . get_post_meta( $order->id, '_some_field', true ) . '</p>';
            echo '<p><strong>' . __( 'Another field' ) . ':</strong>' . get_post_meta( $order->id, '_another_field', true ) . '</p>';
            echo 'Extra: ' . get_post_meta( $order->id, 'my_field_name', true ); ?>
    </div>
    
<?php }
add_action( 'woocommerce_admin_order_data_after_order_details', 'kia_display_order_data_in_admin' );

// WooCommerce 2.3+
function kia_email_order_meta_fields( $fields, $sent_to_admin, $order ) {
    $fields['some_field'] = array(
                'label' => __( 'Some field' ),
                'value' => get_post_meta( $order->id, '_some_field', true ),
            );
    $fields['another_field'] = array(
                'label' => __( 'Another field' ),
                'value' => get_post_meta( $order->id, '_another_field', true ),
            );
    return $fields;
}
add_filter('woocommerce_email_order_meta_fields', 'kia_email_order_meta_keys', 10, 3 );

function kia_display_email_order_meta( $order, $sent_to_admin, $plain_text ) {
    $some_field = get_post_meta( $order->id, '_some_field', true );
    $another_field = get_post_meta( $order->id, '_another_field', true );
    if( $plain_text ){
        echo 'The value for some field is ' . $some_field . ' while the value of another field is ' . $another_field;
    } else {
        echo '<p>The value for <strong>some field</strong> is ' . $some_field. ' while the value of <strong>another field</strong> is ' . $another_field .'</p>';
    }
}
add_action('woocommerce_email_customer_details', 'kia_display_email_order_meta', 30, 3 );

/******************** Backend 2 *************/
/**
* Validate the custom field with custom message.
*/
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');
function my_custom_checkout_field_process() {
    // Check if set, if its not set add an error.
    if ( ! $_POST['my_field_name'] )
        wc_add_notice( __( 'Please enter something into this new shiny field.' ), 'error' );
}


/**
* Save the order meta with field value
*/
add_action( 'woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta' );
function my_custom_checkout_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['my_field_name'] ) ) {
        update_post_meta( $order_id, 'my_field_name', sanitize_text_field( $_POST['my_field_name'] ) );
    }
}

/**
* Display field value on the order edit page
*/
add_action( 'woocommerce_admin_order_data_after_billing_address', 'my_custom_checkout_field_display_admin_order_meta', 10, 1 );
function my_custom_checkout_field_display_admin_order_meta($order){
    echo '<p><strong>'.__('Extra Information').':</strong> ' . get_post_meta( $order->id, 'my_field_name', true ) . '</p>';
}

// HOOKS
//woocommerce_before_checkout_shipping_form
//woocommerce_after_checkout_shipping_form
//woocommerce_before_order_notes
//woocommerce_after_order_notes
//woocommerce_before_checkout_billing_form
//woocommerce_after_checkout_billing_form
//woocommerce_before_checkout_registration_form
//woocommerce_after_checkout_registration_form