Bunnylover, on the detail screen set the preference to 'building'. Keep putting the villagers on a building task until they are trainee's ( this usually takes about 4 or so successful attempts). You can also use a potion, found in the VV3 guides that will encourage exercise. As we know, exercise can make us stronger and maybe even better buliders! For any benefit, let them finish their exercise or their task. If they are gaining skill your bottom bar will read 'your villager improved at construction!' or something like that.

Good Luck!