You just need to fill in the activity to be run in the background (see the screenshot for an example), you are ready to go. Also for Pega 8.1+, this is using queue processor so make sure your steam node is up and running.
I have configured in the same way and connected to an assignment in the flow .Please find the attachment.
As you stated ,I could see one queue processor "pxStandardProcessor" in Admin Studio-->QueueProcessor .But a message is displayed in that window that "Stream service is down. The values displayed on this page may be inaccurate.".