Okay, so, Skin, Steel, and Spirit aren't like other stats, you can't raise them directly. Instead, their value is determined by how many of the Actions in their category you have at least one dot in.
For the sake of example, if someone had a single dot in Skulk and Steady, their Skin would be rated at 2. If they buy another point of Skulk as an Advancement, that doesn't impact their Skin because they already have Skulk.
Strive being moved into a particular category means that there's a forth stat in that category. That means that, immediately, their Resistance is one level higher (because Strive comes with dots in it), but also that they can take it one level higher than most other people.
For a task, you roll a number of dice equal to the number of dots you have in that action. If you have zero dots, you can still roll, but you roll 2d6 and take the lowest (this is why being Bound or Locked is worse than simply having no dots in something, your odds might not be great going in untrained but at least you can try).
As an interaction, remember that you can always spend 2 Stress to Push Yourself and gain +1D to a test. This includes stuff that you have no dots in. (If something is Locked, you can't even do this. If it's Bound, you can Push Yourself to make an attempt, but don't get the +1D).