How do I get an adept builder to become a master builder?

If you have clicked on 'builder' in the individuals profile then you need to keep that individual on track by making sure that every time they stray you take them back to a building to work on it.

After a while their skill will increase through Untrained to Trainee to Adept and eventually to Master, then you can re-direct his skill ability by going back into their profile and clicking on another skill.