In addition to creating the skill, you also need to create some localization entries for the skill in the editor so that you can see what it is ingame. The names in the skill entry point to which entry to use, while the entry itself says what is displayed. The Ref entries for the Description etc aren't used at all; they are just there for your reference.
Levels of -1 scale with the skill user's level. This is mainly used by enemies because they can vary in their level even if they use the same skill.
The .xlsm files aren't necessary to create functioning skills; it's the txt files that are generated that matter. The issue I have with using All.xlsm is that it compiles *everything* and overwrites everything even if you're just adding one entry. The txt files added to your mod can have just a single skill entry and work just fine. I'm not sure if this has changed, but I'm pretty sure there were issues with overwriting existing skills (they didn't apply if I recall correctly).
Lastly, there is the issue of giving access to the skill in the game. There are a few ways to do this, but require some extra work in either other .xlsm files or with the scripts.
