How to use site night vouchers

hi all im a new member to the club and new to caravans as a whole but ive taken out the cc credit card and on my was to earning a voucher my question is this when you book a site through the club i guess you pay up before you go so i was wondering at what stage do i get in and use my free night voucher do i call my chosen site to inform them and they lessen my bill or does everyone pay on arrival to the site and just hand over the voucher and help would be welcome thankyou