To activate backorders for your product, go to Purchase Orders and click the related PO. At the right side of the page, you'll see "Backorder is not activated". Click on it.
After entering the buffer day, click "Activate backorder".
When you activate your backorders, you can go to backorder listings.
Clicking "Show backorder listings" will take you to the Backorders page with a filter of the PO that you created backorder.
To stop backorders, check the article How to stop an active backorder?