When they were linked up to Air Miles I successfuly converted and then used the air miles to book flights. These days it's Avios, but the principle is the same:
online change your vouchers. You'll get an e-mail in a day or so saying they are there for you. Go onto the Avios site and book your flights.
Or if it's Monarch change them online for Monarch e-vouchers. When they arrive, go onto thw Monarch site and book your flights using the e-vouchers in full or part payment
The thing to remember is it's a two stage process