Ordered items can use just the following order system, or use the new Purchase Order. The main difference is the PO system assigns a PO# to the items, all the items will show for just that PO#, and each PO# is for just one vendor.
The ordered items are just items, not related to a PO, so if that is OK with you, you can just put items on order as shown below. That being said, the actual Purchase Order organizes your orders much better. You can put an item on order from here, then go into the PO and assign the part and actual PO#. These PO#'s will all be zero.