Every time you level up, put your three skill points in Honour.
Honour will give you more experience per battle, so you level up faster (and get the achievement sooner). Plus you'll be stronger in battles.
Once you hit 25 or more, press A to accept and you get the achievement.
ALL your skill points should go straight into Honour with no exception. When you get to 75 honour, stop. There is a level cap of 50 so bonus xp will do you no good when you no longer level. If you're going for all awards (last achievement) then you need honour at 75 anyway (along with the other skills).