To use the Cash Back Vouchers: 

Step 1: Go to the Rewards section..

Step 2: Click on the See Vouchers button.

Step 2: To use the voucher, you must activate it before your next purchase. To activate the voucher, slide on the SLIDE TO ACTIVATE button 

Step 3: Just spend as usual and you will receive extra cash back on top of your purchase.

Do note that the usage of the voucher may be subjected to a merchant’s minimum spend.