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'