In the orders area of the application, you will see the Purchase Orders tab. Here there will be one of the columns in the KanBan board that is for draft purchase orders.
A draft order allows you to keep adding products until the PO is ready. After it is ready, you can export the PO as PDF document and send it your method of choice.
After you have sent out the POs outside the application, you can update the status of the PO by dragging over the PO in the KanBan board, or you can change it from the status dropdown from within the purchase order.