The Prometheus-Alertmanager uses the Incoming Webhooks feature of Slack, so first the webhook needs to be set up.

From within slack settings, Go to the Incoming Webhooks page in the App Directory and click “Install” (or “Configure” and then “Add Configuration” if it’s already installed):

You can then configure your new webhook. Choose the default channel to post to, and then add the integration:

This will then give us the Webhook URL we need:

an example configuration of alertmanager.yml is shown below:

route:
 group_by: ['job', 'env', 'alertname']
 # If an alert isn't caught by a route, send it slack.
 receiver: slack_general
 routes:
  # Send severity=slack alerts to slack.
  - match:
      severity: slack
    receiver: slack_general

receivers:
- name: slack_general
  slack_configs:
  - api_url: $WEBHOOK_URL
    channel: '#general'

That’s all now setup, and you can see your firing alerts in Slack. For more information on how to configure Alertmanager, refer: https://bitbucket.org/errigal/prometheus-monitoring-config