One way to receive parts from a purchase order is to create some general receiving transactions, and to choose that purchase order document in the Document field.
However, there is an automatic way that may be more convenient for you.
Navigate to the Document's Detail View
First, go to the purchase order's details page.
Find and click the tab that is labelled "details" ( if it is not open already, as it is shown by default ). On this tab you should see a table detailing which parts are on order and how many have been received so far. Above that table is a button with a pencil icon labelled "receive parts". Click the button and you will be taken to a pre-filled set of forms that represents items that could be potentially received from the purchase order.
The Automatic ReceivingPage is a Formset
See the "Formsets" portion of this page for details on how to work with formsets in Marty 5.
The page linked above also contains general information about working with forms in Marty. That information applies to your auto-receive forms as well.
How Many to Receive?
The "To Store Now" field is pre-filled, and tries to store/receive everything on the order that hasn't been received in past transactions. If you are not receiving the entire order, be sure to change the "To Store Now" field for each part, putting in the number that you are currently receiving.
Don't Worry About the "Greyed-Out" Fields
The "Ordered" and "Yet To Receive" fields are greyed-out because they require no input from the user. If the "SNs" field is greyed out, it means that the part is not serializable, so you don't need to provide SNs for this reception.