/**
 * Add the field to the checkout
 **/
add_action('woocommerce_after_order_notes', 'my_custom_checkout_field');

function my_custom_checkout_field( $checkout ) {

    echo '<div id="my_custom_checkout_field"><h3>'.__('My Field').'</h3>';

    woocommerce_form_field( 'my_field_name', array(
        'type'          => 'text',
        'class'         => array('my-field-class orm-row-wide'),
        'label'         => __('DOB of Person To be Named On Share Contribution Certificate'),
        'placeholder'   => __('DD/MM/YYYY'),
        ), $checkout->get_value( 'my_field_name' ));

    echo '</div>';

}
/**
 * Process the checkout
 **/
add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');
function my_custom_checkout_field_process() {
    global $woocommerce;

    // Check if set, if its not set add an error. This one is only requite for companies
    if ($_POST['billing_company'])
        if (!$_POST['my_field_name'])
            $woocommerce->add_error( __('Please enter your XXX.') );
}
/**
 * Update the user meta with field value
 **/
add_action('woocommerce_checkout_update_user_meta', 'my_custom_checkout_field_update_user_meta');
function my_custom_checkout_field_update_user_meta( $user_id ) {
    if ($user_id && $_POST['my_field_name']) update_user_meta( $user_id, 'my_field_name', esc_attr($_POST['my_field_name']) );
}
/**
 * Update 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( $order_id ) {
    if ($_POST['my_field_name']) update_post_meta( $order_id, 'My Field', esc_attr($_POST['my_field_name']));
}
/**
 * Add the field to order emails
 **/
add_filter('woocommerce_email_order_meta_keys', 'my_custom_checkout_field_order_meta_keys');
function my_custom_checkout_field_order_meta_keys( $keys ) {
    $keys[] = 'My Field';
    return $keys;
}