They will have an online code printed on them.
When you get to check out, it will ask for any vouchers/codes.
You will have to enter each voucher one by one.
As chocolate said, it is better value to use them for deals.
We had a fab long weekend in London last August using the vouchers for hotel and days out. However, having said that, a friend of mine used hers for a holiday last July - through cosmos. Half Board holiday - she only had to put £200 to.
I am saving mine again for a second warm holiday next year.