Hmmm... I explained this once before in another section of the forums. I kinda rushed it, and it\'s probably better suited here anyhow.

Once again, this is just how it looks to me to work... there are quite probably little subtleties I\'ve missed.
First, some basics. As you kill enemies, you gain experience. Once you earn enough experience to reach one hundred percent, you gain progression points. Progression points are what you will need to take to an NPC that can train a skill. I\'m pretty sure you\'ll also need a little bit of tria to purchase training. Or perhaps I have a hole in my pocket.

In the skill window, you see lots of pretty colored bars. The background for these bars is red. The yellow bar is over that, and indicates how much you need to train that skill for the next rank. If you currently have progression points, you\'ll also see a thin blue bar that represents how many progression points you have to spend.
After targeting a trainer NPC and selecting the Train shortcut (or typing /train in your chat window) your skill window should appear. Selecting a skill, and hitting the little \"bag icon\" at the top will spend a progression point on that skill. This will cause the blue bar to decrease, and a some of that yellow bar should turn green.
What you\'ve just bought is what the Player\'s Guide refers to as \"Theoretical Knowledge\". You haven\'t yet ranked up that skill, and you can\'t buy any more training for it at this point.
Now you\'ve got to actually use that skill. As you do, you\'ll see that you\'ve earned practice points. Buying training in your sword skill, and then whacking at a rat will result in something like \"You\'ve earned practice point for sword.\" in your chat window. If you get enough of these, and have turned all the yellow to green, it\'ll tell you that you\'ve ranked up. This corresponds to the \"Practical Knowledge\" part of the skills section of the Player\'s Guide.
If you check your skills window, you\'ll see that this is happily true, and that the green has given way to all yellow once again. You can now take progression points and purchase more training.

Rinse, and repeat.
Once again, I\'m just a fellow player, and could be mistaken here and there. But this is how it seems to work.