1.Create a di.xml under 'etc' directory of your any custom module
<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
<type name="Magento\Checkout\Block\Checkout\LayoutProcessor">
<plugin name="rewrite-checkout-fields" type="Namespace\Module\Model\Checkout\LayoutProcessorPlugin" sortOrder="10"/>
</type>
</config>
2.Create a file called LayoutProcessorPlugin.php file under 'Namespace/Module/Model/Checkout'.
<?php
namespace Namespace\Module\Model\Checkout;
class LayoutProcessorPlugin
{
/**
* @param \Magento\Checkout\Block\Checkout\LayoutProcessor $subject
* @param array $jsLayout
* @return array
*/
public function afterProcess(
\Magento\Checkout\Block\Checkout\LayoutProcessor $subject,
array $jsLayout
) {
$jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']
['children']['shippingAddress']['children']['shipping-address-fieldset']['children']['firstname']['notice'] = __('Please enter first name here');
$jsLayout['components']['checkout']['children']['steps']['children']['shipping-step']
['children']['shippingAddress']['children']['shipping-address-fieldset']['children']['lastname']['notice'] = __('Please enter last name here');
return $jsLayout;
}
}
Note: here i add comment below first and last name you can do for others too.Please accept and vote if use full to you this will help others too.