Open Lucius's Lockbox, recover the Token of Thievery and return it to Lucius in Lakeshire.
We've been training thieves here for years! After all, not every rapscallion is born with the grace and finesse of old Lucius.
Up off the road here you'll find Alther's Mill. I got a chest up there with something important inside. Go get it for me! Bring back that token and you'll receive your Certificate of Thievery!