1. Firstly, we'll set up the controller. Create a src folder in your module's base directory and another folder named Controller inside it.

2. Create MyPageController.php that will hold the route's controller class:

3. The PSR-4 standard states that filenames match the class names they hold, so we will create a MyPageController class:

<?php 
 
namespace Drupal\mymodule\Controller; 
 
use Drupal\Core\Controller\ControllerBase; 
 
/** 
 * Returns responses for My Module module. 
 */ 
class MyPageController extends ControllerBase { 
 
} 

This creates the MyPageController class, which extends the \Drupal\Core\Controller\ControllerBase class. This base class provides a handful of utilities for interacting with the container.

The Drupal\mymodule\Controller namespace allows Drupal to automatically load the file from /modules/mymodule/src/Controller.

4. Next, we will create a method that returns a string of text in our class. Add the following method to our MyPageController class:

/** 
 * Returns markup for our custom page. 
 */ 
public function customPage() { 
  return [ 
    '#markup' => t('Welcome to my custom page!'), 
  ]; 
} 

The customPage method returns a render array that the Drupal theming layer can parse. The #markup key denotes a value that does not have any additional rendering or theming processes.

5. Create a mymodule.routing.yml file in the base directory of your module so that a route can be added to this controller and method.
6. The first step is to define the route's internal name for the route to be referenced by:

mymodule.mypage: 

7. Give the route a path (mypage):

mymodule.mypage: 
  path: '/mypage' 

8. The defaults key allows us to provide the controller with a fully qualified class name, the method to use, and the page's title:

mymodule.mypage: 
  path: '/mypage' 
  defaults: 
    _controller: '\Drupal\mymodule\Controller\MyPageController::customPage' 
    _title: 'My custom page' 

You need to provide the initial \ when providing the fully qualified class name.

9. Lastly, define a requirements key to set the access callback:

mymodule.mypage: 
  path: '/mypage' 
  defaults: 
  _controller: '\Drupal\mymodule\Controller\MyPageController::customPage' 
  _title: 'My custom page' 
requirements: 
  _permission: 'access content' 

10. Go to Configuration, and then Performance in the DEVEOLOPMENT section, and click on Clear all caches button to rebuild Drupal's routes.
11. Go to /mypage on your Drupal site and view your custom page: