This support article explains the steps on creating a purchase order and receiving inventory on a purchase order.
Go to Catalog > Inventory > Purchase Orders
Click the Create PO button
Search for the vendor you want to place a PO with and then click on the contact name (PLEASE NOTE: If you search for a Vendor and none appears, ensure that you have at least one contact for that Vendor).
Save the PO as a Draft using the Save as Draft button on the right. Next click on the Products section and the Products Search button (blue)
Once you have searched and added the SKUs to your PO, select the "Close" button.
Place the PO
To Receive on your PO, navigate to the Receiving section of the PO and click the Receiving button (green)
Select the line items and quantities for each line and then select Save and Close to receive your items.