Hi,
As mentioned above, you simply need to start the booking process for a 1-night stay with points.
At stage 2 of the process, the system will automatically offer to apply the certificate instead of points, provided of course that you booked a cat 1-4 hotel
Trying to book a multi-night stay could be more tricky and require a call, but just book as single nights and you will be fine.
Worked like a charm for me in April.
Cheers,
GenevaFlyer