I don't ever remember getting order to do anyting via a memo - always always always I received orders verbally via the chain of command. The ONLy time I received "orders" via paper was when they were travel orders...
There were a few times I had to give orders in writing. It was ALWAYS on official letterhead, and in the presence of the 1st Sgt. The person receiving the order signed an acknowledgement that they had received and understood the order and the 1st Sgt signed as a witness.