Once the extension is installed and activated you can navigate to Downloads → Settings → Extensions → Slack and you will want to first create an Incoming Webhook for your Slack Team.
This allows EDD Slack to send Notifications on your behalf, meaning it can only send Notifications to the same Channels that you can. If you have access to a Private Channel it can send Notifications to that Private Channel, but it cannot send Notifications to Private Channels which you do not have access to.
While creating an Incoming Webhook Integration Slack will ask you which Channel you would like to post to. Think of this as a “Default Channel”. For each Notification that you create in EDD Slack you are able to designate different Channels if needed.
Once you have generated an Incoming Webhook, enter it into the “Default Webhook URL” field and click “Save Changes”. Now you can proceed to Create Notifications!