There are two ways that a character can learn a new (previously unknown)
The First WayAny time your character uses an unknown skill, and your roll is exactly what was required for success, your character learns the skill, at the default level (it is no longer an unknown skill, but the chance of success is still the same). This is now a partially known skill.
For such skills, the three character sheet columns for skill levels would be used as follows: the first column for the skill level the character would have if they properly knew the skill (calculated from their attributes, as described in Selecting Initial Skills), the second column for the default usage level (1/8, 1/4 or 1/2), and the third column for the product of the first two columns (the actual value which will be checked against).
Every time practice would normally increase the skill level by one point (as described under Improving Skills), the default usage level is increased by one step instead, from 1/8 to 1/4 to 1/2 to 1. Once it reaches 1, replace the 1 in the second column with a 0, which now represents the total points earned by practice and training. At this point, the character knows the skill as well as if they had been created with it in the first place.
The Other WayA character may learn a new skill by receiving training from another character (PC or NPC) who knows the skill. It is assumed that most of the skills your character started with were learned in this way.
To learn from another character, the teacher must fully (not partially) know the skill being taught. They must also have a Teaching skill level of at least 75, as teaching the basics right from the beginning is more difficult than adding to existing knowledge.
Then, simply follow the rules for the Training skill, with the student's skill level beginning at 0. Once the student's skill level reaches what it would be if they properly knew the skill (calculated from their attributes, as described in Selecting Initial Skills), they are considered to know the skill as well as if they had been created with it in the first place.
While learning a skill in this manner, it is recommended that the three character sheet columns for skill levels be used as follows: the first column blank (or 0) to remind you of the ongoing training, the second column for the total points earned through training, and the third column blank (or just a copy of the second column). After the skill is learned, the columns revert to their normal usage.