Instructions:Buy a 16-25 Railcard online now and you will get 10% off the usual £26 (paying £23.40)
Enter the promotional code on the first page of the signup process. If you are renewing then after logging in press edit next to the 'Railcard Type' on the purchase summary and enter the code.
This offer is only available online and is not available in stations.
[Note that a 3yr option is also available (which the code is not valid on), which works out as better value if you need it (£21.67/year) - an even greater saving when you consider that the railcard prices go up every year as well. The 3 year card is only available to purchase up to the day before you turn 24 though.]