Here is how you can add notes (staff notes, or comments) to an existing purchase order.
1. Click on the POs tab in the top menu and then click on the Purchase Order ID which needs the note.
2. Click on Add a comment towards the top of the page, next to the Purchase Order ID and add whatever comments you would like and then click Add Comment
3. The comments that you add will show up in the PO history tab on that page