Once it had been learned, the following skills are opened up:
Musicmanship can be learned actively or passively after getting the first 30 points from an NPC Bard.
Double-click any musical instrument to have a chance at gaining in skill. If a character has completed training other bard skills, this is the quickest method.
Musicianship has a chance to raise anytime one of the other bard skills is used. Most likely, Musicianship will be at the desired level before other bard skills have completed training so it is best to focus on those first.