Create a custom permission in Drupal 8

Example with a module 'mymodule'

Step 1.

Create if not exist yet, mymodule.permissions.yml (root of mymodule)

Add your new permission like:

mymodule test_permission:

    title: 'My Test Permission'

    description: 'The description'

    restrict access: false

Step 2.

On your mymodule.routing.yml

mymodule.home:

  path: 'test/home'

  defaults:

    _controller: '\Drupal\mymodule\Controller\Test::home'

    _title: 'My test Home'

  requirements:

    _permission: 'mymodule test_permission'

Set permissions on : /admin/people/permissions

Permissions AND condition

Must have all

Example:

  requirements:

    _permission: 'mymodule test_permission, mymodule permission'

Permissions ORcondition

Must have at least one

Example:

  requirements:

    _permission: 'mymodule test_permission+mymodule permission'