Go to "Workflows" and click "Inspiration Library"
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Here you will see all the templates on the most commonly used Salesforce objects ( opportunity, lead, case, task, campaign). Click the template you want.
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As soon as you click the template of your choice, a workflow will open up. Simply add the workflow name and recipient to set your Slack alert.

Note that, you can customize these templates by adding conditions, fields, action buttons as you want.