Confirm with US Airways that you have enough miles for the person who will be using your miles for travel.
Confirm the name of the passenger who will be taking the trip.
Book the flight by redeeming miles in your account, making sure to book the ticket under the passenger's name and not your own.
The miles will be automatically attributed to the trip you booked under the passenger's name. Keep in mind, you cannot change the name of the passenger once the flight is booked, so make sure you get it right the first time.