To order items in chosen order, you use the "Curate" functionality. Note that for your items to appear in manual curated order as a default, you need to do two things:
Select "Curated" order as default for your list. See How.
Use "Curate" functionality to set the order of your items
Manual Curation of Items
On your list page, click on "Curate" Tab in the List Toolbar. You will see this tab only if you are the owner of the list or you have curator privileges for it.
Your list will now switch to curate tab and be in drag and drop mode. Simply order your list by dragging and dropping boxes in appropriate position and the list will be automatically saved as your place them in the new order.