Things you may want to cover: Make sure the nerdgeschoss development is running correctly (https://github.com/nerdgeschoss/development-environment) and follow the usual setup steps outlined there.
To test notifications join the test-channel on the NG workspace in slack. That is where messages arrive in development.
To test notifications to private channels (e.g. HR channel ), create a new private channel, add the Nerdgeschoss App to the channel as an integration and insert the channel ID into the credentials YAML file in Rails.
To test notifications via the legacy webhook join the flink_testing channel. It is not possible to create new custom webhooks. They will be deprecated eventually.