To enable Slack notifications for certain events in Humanitec, users need to create a Slack workflow and a webhook in Humanitec. The process involves creating a new workflow in Slack's Workflow Builder, configuring it with variables from the Humanitec webhook request, adding a step to send a message to a channel, publishing the workflow, and copying its URL to set up the webhook in Humanitec. The user must also have an App set up in Humanitec, a Slack organization using a paid plan, and access to the Webhooks section of their application's settings screen. By following these steps, users can receive notifications in Slack when a deployment is finished in Humanitec.