settings.py

INSTALLED_APPS = (
   (snip)
   'django_auth_custom_example', # <--
)
   
MIDDLEWARE_CLASSES = (
   (snip)
   'django.contrib.auth.middleware.AuthenticationMiddleware',
   'django_auth_custom_example.middleware.CustomAuthMiddleware',  # <--
   (snip)
)

AUTHENTICATION_BACKENDS = (
   (snip)
   'django_auth_custom_example.backends.CustomAuthBackend',  # <--
   'django.contrib.auth.backends.ModelBackend',
   (snip)
)

# for django REST framework
REST_FRAMEWORK = {
   'DEFAULT_AUTHENTICATION_CLASSES': (
         'django_auth_custom_example.authentication.TokenAuthentication',  # <--
         (snip)
         'rest_framework.authentication.SessionAuthentication',
   ),
   (snip)
}

AUTH_CUSTOM_SERVER_URI = 'http://example.org/v1/auth'  # <--