Using Slack as an alerting channel is pretty simple.
First you'll need to create an Incoming WebHook integration in Slack by selecting a default channel for your alerts. You'll see a WebHook URL generated by Slack. Copy that URL and then in Checkly, go to Alert Settings -> Add More Channels -> Slack to use that URL to add it as an alerting channel:
You have the option to customize the default channel you've set in Slack by setting a channel name while creating your Alert Channel. You can use a #public-channel-name, @username for Direct Messages or a channel ID. If you omit this field, the message will arrive in the default channel you've picked while creating the Incoming WebHook URL in Slack.
By customizing the channel name, alerting options & checks for that alert channel, you can use a single Incoming WebHook URL for multiple scenarios.