Skill is the most important attribute a character has. It is a measure of experience. Every ten skill is considered a stage. Each stage gives you an occupation, an increase by 1 in will power and awareness, and an increase in stamina and bonuses depending on the occupation gained.
Often the most important consideration when determining the effectiveness of a character is how many stages they have. At 0 - 2 stages, a character is considered "green". At 3 - 4 stages, a character is considered "professional". At 5 - 9 stages a character is considered "seasoned". And at 10+ stages a character is considered "legendary". Often when interacting with another, you won't be able to determine exactly how many and what kind of stages another being has, but you should be able to determine what category they fall in. For example, you may ask the referee how tough a potential opponent seems. The referee may reply, "He seems to be a seasoned combatant." If you were only 1st stage at the time, you would probably make a hasty exit.
As a character lives his life, he will gain skill depending on experience and what he learns. Gaining skill depends greatly on what occupation the character is working towards. At the beginning of each stage, the character should let the referee know what he wants his next stage to be. Then, a character will only gain skill if the referee feels that he has learned or gained experience in the occupation the character is trying for.
The actual rate of skill earning is left up to the referee. There is no rigid way to determine how much skill should be awarded based on specific accomplishments. The rate that a character earns skill should be slower when they reach higher stages. Usually green characters will gain skill quickly, say two to four skill for every evenly matched fight they win. Professional's will earn skill slower, say one to two skill for every evenly matched fight they win. Once you reach seasoned, each stage will result in slower skill earning for the following stage. Remember, beings that are seventh stage or higher are quite rare, so it should be very difficult to obtain.
The diversification of occupations also plays a part in the rate of skill earning. If a seasoned character has a very focused set of occupations, then it will be more difficult to learn more skill towards that focus then it would be to learn something new. For example, a character with one stage of mercenary, one of weapon specialist, two of infantry, and one of scout who is gaining skill towards weapon specialist would earn skill slightly faster than a character with three stages of weapon specialist, one of infantry, and one of mercenary. This is a slight difference and shouldn't matter until the character reaches three or more of the same occupation.
But, as always, the referee has the final say. While consistency is recommended, the referee can choose to award skill at any rate they see fit.
The occupation of scientist has a different method of earning skill. See Gaining Skill in Scientist. Also, the occupations of Lunatic and Mentalist have unique characteristics in regards to skill that are explained in their descriptions.