Several people told me I should book a flight as soon as possible,
because there are very few seats available for these vouchers

I've used Northwest Airlines vouchers before, nearly a year later, with
no problems. In fact, I voluntarily bumped myself off the overbooked
flight for a voucher for another flight within one year, and an upgrade
to first class for the original destination later that same day.


How can you actually use the free ticket?

Just make your reservation, and when it comes to the part where it's
time to tell them how you're going to pay, tell them you're using a
voucher. Or tell them on the front end; makes no difference.