Enter your code at the payment stage when booking online, or quote it when booking by phone.
A few rules apply to every code:
One offer per booking. Codes can't be combined with each other, with an on-board voucher or with a partner voucher.
Codes have an expiry date and may apply only to certain journeys or departure stations; the email or page that gave you the code says which.
Codes can't be applied to a booking that's already been made.
If your code is rejected, check its expiry and the journey it applies to, then contact the reservations team with the code and the journey you're trying to book.