Your voucher will have a voucher code and a security code, which can be used to book online.
Just go through the booking process as usual, and during Checkout you will reach a 'Review' page where you can 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.
You should then continue through the Checkout process and complete your booking. Once this has been completed, you'll receive a booking confirmation email. These often go into junk mail folders, so please check there if you don't see it!
Visit www.mendip.me/myvoucher for more info about your voucher and links to book.