Macworld:

If you already have the Workflow app, you just need to open the App Store and hit the Updates section. Shortcuts comes as an update to Workflow, and replaces that app.

If you don’t have it yet, you’re missing out on one of the best parts of iOS 12! Just click Shortcuts from the App Store to grab it and get started unlocking the real potential of Siri.

The app obviously requires iOS 12 so don’t bother to download until you’ve upgraded.