How to customize the comments form in drupal 8

Step 1:- I would do it:

1) Create new file: testmodule.info.yml.

name: testmodule

description: Testmodule

type: module

core: 8.x

2) Create new file: testmodule.module

<?php

function testmodule_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  if ($form_id=='comment_comment_form') {

    //add class subject

    $form['subject']['widget']['0']['value']['#attributes']['class'][]='newclass_subject';

    //add placeholder subject

    $form['subject']['widget']['0']['value']['#attributes']['placeholder'] = t('Placeholder Subject');

    //add class body

    $form['comment_body']['widget']['0']['#attributes']['class'][]='newclass_body';

    //add placeholder body

    $form['comment_body']['widget']['0']['#attributes']['placeholder'] = t('Placeholder Body');

  }

}