Simply put, you just need to enter the amount of money that you want on the voucher. The Rewards Gateway system then works out how much you'll need to pay and take that off your purchase of the voucher.
For example, you want an instant voucher with £100 on, and the offer has a 4% discount. You enter £100 as this is how much you would like on the voucher.
Once the voucher is added to your basket, you get £100 value on the card but only pay £96 for it!