/* 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" class="my_new_field"><h2>' . __('My Field') . '</h2>';

woocommerce_form_field( 'my_field_name', array(
    'type'          => 'text',
    'class'         => array('my-field-class form-row-wide'),
    'label'         => __('Fill in this field'),
    'placeholder'   => __('Enter something'),
    ), $checkout->get_value( 'my_field_name' ));

echo '</div>';
}

// Hook in
add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );

// Our hooked in function - $fields is passed via the filter!

function custom_override_checkout_fields( $fields ) {

 $fields['order']['order_comments']['placeholder'] = 'My new placeholder';

 $fields['order']['order_comments']['label'] = 'My new label';
 return $fields;
}