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You don't need to do anything until you are ready to cash them in. You'll have to coordinate a bit, depending on the airline, so that you can make the transfer and do the redemption without risking the seats disappearing while the transfer is happening. Some airlines (CO, DL) are instant transfers, while others (NH) may take a couple days, so it can be difficult.

When you are ready to use them you'll have to find a reward seat on the appropriate airline and then make the transfer from AmEx to the airline and then actually issue the ticket. It is pretty straightforward, except the part about finding availability for a reward seat.