It's pretty straightforward, really.
Choose the reward option you want (e.g. Prestige car for two weekend days), click on on that button/option, the website transfers you to booking and includes the PC code. You make your booking. The reservation shows the value of your e-coupon (i.e. points redeemed) giving your the $ value and shows the taxes and fees and off you go. I'm confused as to why you seem to find it difficult?