Gains in proficiency come with experience using the skill. Experience is counted for each turn that a skill use is successfully completed. If a skill is used more than once per turn, only the first success will count towards experience. Projects which take multiple turns to complete, such as shipbuilding or castle construction, only count towards experience when the project is finished. For example, a master shipbuilder will be able to construct a galley somewhat faster than an apprentice.
Some skill uses benefit more from experience than others.