Open a List
Switch to "Prepare" mode
Choose "Select a Store" (!), go back to List tab
Press "Arrange", choose "Order Manually"
Now each item has a hash tag Ordering Control on the right
Grab the Control and drag the item
If you choose a Specific store (Custom store), you'll see "Like manually ordered" option only (to avoid confusion of different orders for the same items).
In order to arrange items at a Specified store please use "As Checked-Off" sort option.
This sort lets you arrange items in the order in which items are checked-off. It moves checked items to the bottom of the list, and it shows unchecked items in the order they have been checked during previous shopping trips with this list.