add_filter( 'woocommerce_checkout_fields' , 'monocommerce_reorder_checkout_billing_fields' );
add_filter( 'woocommerce_checkout_fields' , 'monocommerce_reorder_checkout_shipping_fields' );

function monocommerce_reorder_checkout_billing_fields( $fields ) {

    // remove unwanted fields
    unset($fields['order']['order_comments']);
    unset($fields['billing']['billing_company']);
    unset($fields['billing']['billing_phone']);

    // reset classes
    $fields['billing']['billing_first_name']['class'] = array('');
    $fields['billing']['billing_last_name']['class'] = array('');
    $fields['billing']['billing_postcode']['class'] = array('');
    $fields['billing']['billing_email']['class'] = array('');

    // set the placeholders
    $fields['billing']['billing_first_name']['placeholder'] = 'First Name';
    $fields['billing']['billing_last_name']['placeholder'] = 'Last Name';
    $fields['billing']['billing_address_1']['placeholder'] = 'Address Line 1';
    $fields['billing']['billing_address_2']['placeholder'] = 'Address Line 2';
    $fields['billing']['billing_email']['placeholder'] = 'Email';
    $fields['billing']['billing_postcode']['placeholder'] = 'Zipcode';

    // reorder everything
    $order = array(
        "billing_first_name",
        "billing_last_name",
        "billing_address_1",
        "billing_address_2",
        "billing_country",
        "billing_city",
        "billing_state",
        "billing_postcode",
        "billing_email"
    );

    foreach($order as $field){
        $fields["billing"][$field]['label'] = '';
        $ordered_fields[$field] = $fields["billing"][$field];
    }

    $fields["billing"] = $ordered_fields;

    return $fields;
}

function monocommerce_reorder_checkout_shipping_fields( $fields ) {

    // remove unwanted fields
    unset($fields['shipping']['shipping_company']);
    unset($fields['shipping']['shipping_phone']);

    // reset classes
    $fields['shipping']['shipping_first_name']['class'] = array('');
    $fields['shipping']['shipping_last_name']['class'] = array('');
    $fields['shipping']['shipping_postcode']['class'] = array('');
    $fields['shipping']['shipping_email']['class'] = array('');

    // set the placeholders
    $fields['shipping']['shipping_first_name']['placeholder'] = 'First Name';
    $fields['shipping']['shipping_last_name']['placeholder'] = 'Last Name';
    $fields['shipping']['shipping_address_1']['placeholder'] = 'Address Line 1';
    $fields['shipping']['shipping_address_2']['placeholder'] = 'Address Line 2';
    $fields['shipping']['shipping_email']['placeholder'] = 'Email';
    $fields['shipping']['shipping_postcode']['placeholder'] = 'Zipcode';

    // reorder everything
    $order = array(
        "shipping_first_name",
        "shipping_last_name",
        "shipping_address_1",
        "shipping_address_2",
        "shipping_country",
        "shipping_city",
        "shipping_state",
        "shipping_postcode",
        "shipping_email"
    );

    foreach($order as $field){
        $fields["shipping"][$field]['label'] = '';
        $ordered_fields[$field] = $fields["shipping"][$field];
    }

    $fields["shipping"] = $ordered_fields;

    return $fields;

}