Call them at evening time (or avoid office peak hours, i.e. 0900~1000 & 1200~1400), book your flight but don't pay the miles yet as you can hold your seat for a month if CX or KA flight. Call again before the deadline & the time you sure that you can fly (even not sure, you can change for other date within a year of your ticket issue date)! Provide your Cr. card no. for the taxes. That's it & good luck! Enjoy your very 1st reward flight!