Log into your QFF account. On the booking page, when the flights become available, put in the dates and route you want - and up will come the revenue fares available for purchase, and the award seats available for each class. Pick the ones you want.
Note - award seats generally become available 353 days before departure. J and F award seats go pretty quickly, so get in early. It might pay to book the two flights, there and back, as one way award tickets, as if you wait for the seats to become available for the return trip, any award seats for the outbound leg may already have been snagged by others.