One way to pick parts for a work order is to create some general inventory transactions, and to choose that work 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 work order's detail page.
Find and click the tab that is labelled "Picked Items". On this tab you should see a table detailing what has been picked so far and what is still required. Above that table is a button with a pencil icon labelled "pick". Click the button and you will be taken to a pre-filled set of forms that represents picks for each item required by the work order.
The Automatic Pick Page 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-pick forms as well.
How Many to Pick?
The "To Pick" field is pre-filled, and tries to pick as many as possible from the inventory to fill the shortage on the work order. If you need 5 of some item for the WO, and there are 3 in inventory, "To Pick" will be 3. If you need 5, and there are 100 in inventory, "To Pick" will be 5. You can adjust these numbers as you see fit.
Don't Worry About the "Greyed-Out" Fields
The "Required" and "Short" 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 pick.