All you have to do to earn this achievement is complete 27 quests in the Orsinium DLC.
Completing the Main Quest for the Orsinium DLC (and earning the associated achievement 'Kingmaker') requires the completion of 9 quests and 3 side quests. The remaining 15 side quests can be any that take place within Wrothgar (the geographical setting of the DLC).
Alternatively you could just complete 27 side quests, a comprehensive list of which can be found here:http://en.uesp.net/wiki/Online:Wrothgar
You can keep track of how many quests you have completed overall in the in-game achievement tracker (you can access this by pressing the
button, scrolling down to 'Journal', then 'Achievements' and then 'Orisinium'). Once you complete 27, the achievement is yours!