In the Automation tab, you can see all your automation workflows. Ones that are active are at the top, and ones that are PAUSED or a DRAFT (meaning they were created but never activated) list their status in orange below.
To activate a workflow that is either PAUSED or a DRAFT, simply locate it in the list and click the arrow next to View Messages. You'll see the option to Edit or Activate - click Activate and your workflow will be live and start sending to customers.
To Pause an active workflow, locate it in the list and click the same arrow - the option to Edit or Pause will appear. Click Pause and your workflow with stop sending to customers.