1. Permissions are stored in a permissions.yml file. Add a mymodule.permissions.yml to the base directory of your module.
2. First, we will need to define the internal string used to identify this permission, such as view mymodule pages:

view mymodule pages: 

3. Each permission is a YAML array of data. We will need to provide a title key that will be displayed on the permissions page:

view mymodule pages: 
  title: 'View my module pages' 

4. Permissions have a description key to provide details of the permission on the permissions page:

view mymodule pages: 
  title: 'View my module pages' 
  description: 'Allows users to view pages provided by My Module' 

5. Save your permissions.yml and edit the module's routing.yml to use the permission for controlling access to a route.
6. Modify the route's requirements key to have a _permission key that is equal to the defined permission:

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

7. Go to Configuration and then to Performance in the DEVELOPMENT section and click on Clear all caches to rebuild Drupal's routes.
8. Go to People and then to Permissions to add your permission as the authenticated user and anonymous user roles for My module!:

9. Log out of your Drupal site and view the /mypage page. You will see the content, and will not receive an access denied page.