Training is an activity one might undertake during downtime. You may train in one of three areas to gain a proficiency, but be warned; This is expensive and slow.
Any character so willing can attempt to learn a new language. To do this, the character must have access to someone fluent in the language willing to teach them, at a cost of 1 GP per day. The time taken to learn a language varies, but ranges from 200 days for a common language, and is higher or lower depending on factors about the language (such as vocabulary range and requiring additional tongues to pronounce certain words correctly).
Any character may gain a skill proficiency through training under a master. A trainer must, themselves, be proficient with the skill, and must be willing to take the character on as an apprentice. This process generally takes 150 days, and standard payment for this service is 2 GP per day. To “show promise”, and thereby qualify as a fitting student, a character must have no less than 13 in the ability score primarily associated with the Skill in order to commence this process.
In some circumstances, it may be possible to raise an Ability Score through downtime training. To do this, a trainer must be found who is willing to train the character, the trainer must have an appropriate venue in which to train, and may only raise an ability to a maximum of five points below their own (i.e. A character with 17 Charisma may only train someone up to an ability score of 12). This process takes 240 days (or one full year, barring one day per week break) for each point trained this way, costing 5 GP per day.