Your voucher will have unique Voucher and Security codes, which are used to book activities online.
Just go through the booking process as usual, and during checkout you will reach a 'Review' page where you redeem your voucher/s. Enter the Voucher Code and Security code. If your voucher is for multiple people, you can select how many uses you would like to redeem from it at this point.
Continue through the checkout process and complete your booking. You'll then receive a booking confirmation email. This may go into your junk mail folder so please check there if you don't see it in your Inbox!
Visit www.mendip.me/myvoucher for more info about your voucher and links to book.