The high-level steps are:
Plan your migration with your team and stakeholders
Migrate your content, current workflow state and users using the Confluence Cloud Migration Assistant
Apply the cloud workflow to the newly migrated space (after you have it prepared in a test space)
If you are using a custom workflow, you will need to recreate your workflow using the workflow builder or the JSON editor. If the custom workflow is used in several spaces, simply copy the workflow JSON and paste it into the JSON editor in the destination space Document Management dashboard. There is no option to add and manage a workflow at global level.