Simply select your preferred flight on www.lufthansa.com and enter your discount voucher code at the end of the booking on the payment screen. The net flight fare (excluding taxes and surcharges) will be reduced up to the maximum of the discount voucher value.
Extract of the terms & conditions:
The voucher can be redeemed between 09/02/2015 and 09/02/2015.
Travel must be between 09/02/2015 and 31/08/2015.
Vouchers are only valid for itineraries where all flights are operated by Lufthansa and Lufthansa Regional Partners (incl. Lufthansa City Line).
The voucher is only valid for flights departing in Germany and can only be redeemed at www.lufthansa.com.