Receiving an order in the military

[huskyteer]

I thought I knew my militaria, but I keep running into problems over the nitty-gritty aspects. I have a major receiving orders to select a number of soldiers for a mission, and I realised I don't know how the order would be given. Written orders in an envelope, over the phone, or in person at a briefing? Is this sort of order even realistic?

Setting: alt-universe, but based on British army, present day, operational in a desert.
Searched: "how are orders given in the military", "military mission orders", "chain of command orders".

Thanks!