From Shopify interface

Custom workflows, depending on the type of trigger, are started from different parts of the Shopify admin interface:

• If the workflow starts with the Custom trigger triggered, it can be started from the Flow Companion interface by the Actions → Start custom workflow action.
• For other types of triggers, the workflow is started
  • from the list of items when we select one or more rows
  • from the item form

See also:

• Custom workflow trigger