Simply enter the known quantities and part numbers.
To delete an item, clear the QUANTITY and PART NUMBER fields.
Digikey is apparently out of stock on some parts so after the "fast order" warning about those comes up you have to hit the Part Search link (look for "If you need to find a part number to enter, click Part Search") above the "ADD LIST" buttons and then click on "Display Order" from the bottom of that page.