To configure Slack notifications, the first thing you need is a Legacy Slack API token, this can be found here. Click Create Token next to your user. You'll need to be logged in to Slack.
The second is the name or ID of the Channel you want your notifications to go to, if you're using the name then you need to precede your room name with the pound sign (#). So if your room name is 'ops' then in the Server Density UI you would input: '#ops'.
Once you have these details you can input them into your notification preferences by clicking your name in the top left, then selecting Notifications. Simply select Slack from the dropdown list to enter your details.
Once this is done, when you add or edit your alert the Slack Notification will appear in the list of notifications available:
Important: if you've selected "Only direct messages & highlight words" in the default notification settings you may miss Server Density alerts unless you check the selected room regularly. To get around this you should setup a notification override (example below) for the room we're sending alerts to.