Once I mark apparel that 'needs to be ordered', where can I see the list of items that I need to order?

Once you mark a line item as 'Needs Ordering', in the 'Purchase Orders' section, you will find all of your purchase orders and their current status (i.e. Goods ordered, Arrived, etc.). 

From here, you can choose an existing PO to order goods or create a completely new PO.