To send Slack direct message notifications, you will need both Courier and Slack accounts. If you don't have accounts already, sign up before proceeding.
You also need permissions in your Slack account to create a Slack app, configure that app, and add it to a Slack workspace. If you do not have access already, you can create your own free Slack workspace.
For a detailed step-by-step guide, check out our Slack getting started guide.