Acquiring Non-Weapon Proficiencies and Bonuses Once a character has as many points in that skill as the stat it is based on, the skill CAN be learned, provided an open slot exists.
Once a player has learned a proficiency, each additional SUCCESSFUL attempt provides an additional point to that skill. When the character has earned as many success points equal to the stat it is based on multiplied by the number of proficiency slots already in the skill, he may then increase the proficiency, provided an open one exists.

For example:
 
 

Janus the archer wishes to learn the skill of Bowyer/Fletcher, but has never had any training in it. He decides to go to the local ranger Kemille for some lessons. After coming to an agreement that Janus will do chores and errands for Kemille for two weeks, she will train Janus in the art of Bower/Fletcher during that time. Janus trains with Kemille for those two weeks and has now gained 10 points toward that skill, however the ranger has no more time to spare and must be off. Bowyer/Fletcher being based on Dexterity with a minus 1 to skill checks would mean that Janus needs a total of 18 points to acquire the proficiency and now roughly has a skill of 9 when attempting to use it (the minus 1 coming from the proficiency). When Janus practices making himself a new bow he fails twice but on his third try succeeds, this has taken him a total of 3 weeks to find the right materials and create three bows. During this time each failure adds 1 point to his total, as does the success for a new total of 13. As Janus is working his new bow, the ranger returns and finishes his training giving him a total of 18 points, enough to learn the new proficiency which has now taken him a total of 6 weeks to learn. Now then, if he wished to put another slot into that skill, giving him a +1 bonus in it, he would need to make 18 more successful proficiency checks. If he wished for a +2 bonus (triple proficiency) he would now need 36 additional successes. +3=54, +4=72 successes and so on.

Of course this does make it easier for characters with lower stats to learn skills sooner, but they are not as good in it either and successes are harder to come by.