I want to know the answer to the following question:
Who must sign these orders for them to be valid?
All the background I'm provided with is that "Under the Westminster system's parliamentary customs and practices, however, the monarch and viceroy must generally follow the advice of his or her ministers in Cabinet, including the prime minister and minister of national defence, who are accountable to the elected House of Commons.". Is that enough to answer the question?
Answer: no