Go to "Workflows", and click the button "Create Workflow". Mention the "Workflow Name" to get started.
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Select the "Opportunity" object from the dropdown. "Stuck in Stage" rattle works only on this object.

After selecting the "Stuck in stage" type of workflow, you create a condition on a specific stage and mention the duration after which you want to get notified. This is a required filter for creating such workflows. Post this you can add as many other conditions as you want.

Customize the Slack message by including any fields, emojis, GIF, etc.
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Note that you can hit "Send Sample Message" to view how your configured message would look like in Slack.

Add an action button to let recipients push changes in Salesforce from Slack. Read more about action buttons here.

The last step is to add the recipient. You can select any Slack channel, or send a DM and group DM to anyone in your organization.

Note that, you can dynamically route the alerts to object owners, their managers, etc.

Hit the "save" button to save your rattle.