If you have a paper voucher, then go to the NowTV website with your internet browser and select My Account > Passes & Vouchers whilst signed in with your nowtv account details.
Next to the entertainment pass you should see a green apply voucher button, click on it and follow the on screen instructions where it will ask you to enter the voucher code.
If you have an electronic voucher pre loaded onto a nowtv stick or box, then again go to the Passes & Vouchers section, where once you setup & activated the stick or box you should hopefully see towards the top of the page a link saying something like Vouchers waiting for You, which you need to click on to access the voucher.
Should you be on an existing entertainment offer with more than 30 days remaining then you want be able to redeem & start the electronic voucher or the paper code voucher where you will not see the green apply voucher button next to your entertainment pass.
Once you have 30 days or less on an existing offer you should be able to redeem & start both styles of vouchers.