It's in the downtime section. Takes 250 days at 1 gp/day to learn a new skill/language. Doesn't say whether those days have to be consecutive or if you can just say "In between adventuring, I go back to Blacksmithing school for a couple of weeks!" and let it gradually accumulate. I'd probably let my crew get away with that, instead of telling them they have to take most of a year off to learn a new skill or something.
[Actually, "proficiency with a tool set", but I'd let that roll into skill training, too.]