https://www.drupal.org/project/commerce_checkout_order_fields. Just enable it, go to /admin/commerce/config/order-types/default/edit/form-display, scroll down, enable 'Checkout' view, drag/drop fields on the Checkout view, and then go to /admin/commerce/config/checkout-flows/manage/default and add 'Order Fields: Checkout' pane to a step you want. Enjoy