Proficiencies

Most of what a player character can do is defined by his race, class, and ability score. These three characteristics don't cover everything, however. Characters can have a wide range of talents, from the potent (and intricate) arts of magic to the simple and mundane knowledge of how to build a good fire. The character's magical ability (or lack thereof) is defined by his class. Lesser abilities, such as fire building, are defined by proficiencies.
A proficiency is a learned skill that isn't essential to the characters class. A ranger, for example, may find it useful to know something about navigation, especially if he lives near an ocean or sea coast. On the other hand, he isn't likely to suffer if he doesn't know how to navigate; he is a ranger, not a sailor.
Proficiencies are divided into two groups: weapon proficiencies (those related to weapons and combat) and non-weapon proficiencies (those related to everything else).
All proficiency rules are additions to the game. Weapon proficiencies are tournament-level rules. Proficiencies are not necessary for a balanced game. They add an additional dimension to characters, however, and anything that enriches characterization is a bonus. If weapon proficiencies are in your game expect them to apply to all characters, including NPCs.
Once a proficiency slot is filled, it can never be changed or reassigned.

Acquiring Proficiencies

Even newly created, 1st-level characters have proficiencies. The number of proficiency slots that a character starts with is determined by his group, as shown in Table 34. Each proficiency slot is empty until the player "fills" it by selecting a proficiency. The character's Intelligence score can modify the number of slots he has, granting him more proficiencies. In both cases, new proficiencies are learned the same way.
Thereafter, as the character advances in experience levels, he gains additional proficiency slots. The rate at which he gains them depends on the group he belongs to. Tables 34a & b lists how many weapon and non-weapon proficiency slots the character starts with, and how many levels the character must gain before he earns another slot.

Training

Like all skills and abilities, proficiencies do not leap unbidden and fully realized into a characters mind. Instead, a character must train, study, and practice to learn a new proficiency. However, role-playing the training time needed to learn a new skill is not much fun. Thus there are no training times or study periods associated with any proficiency. When a character chooses a proficiency, it is assumed that he had been studying it in his spare time.
Consider just how much spare time the character has. The player is not role-playing every second of his character's life. The player may decide to have his character spend a night in town before setting out on the long journey the next day. Perhaps the character must wait around for several days while his companions heal from the last adventure. Or he might spend weeks on an uneventful ocean voyage. What is he doing during that time?
Among other things, he is studying whatever new proficiencies he will eventually learn. Using this "down time" to handle the unexciting aspects of a role-playing campaign lets players concentrate on more important (or more interesting) matters.
Another part of training is finding a teacher. Most skills are easier to learn if someone teaches the character. The DM can handle this in several ways. For those who like simplicity, ignore the need for teachers—there are self-taught people everywhere in the world. For those who want more complexity, make the player characters find someone to teach them any new proficiency they want to learn. This can be another player character or an NPC. Although this adds realism, it tends to limit the PCs adventuring options, especially if he is required to stay in regular contact with his instructor. Furthermore, most teachers want payment. While a barter arrangement might be reached, the normal payment is cash. The actual cost of the service depends on the nature of the skill, the amount of training desired, the availability of tutors, and the greed of the instructor.