After I read about how to use active skills, I understood it. First, I am required to practice what triggers the skills I will use. If it's true, it will be able to use, but if I can't properly trigger the skill, it can't be used.
I've tried that, and it turned out to be easy. I just need to remember what triggers it, such as movement, stance gesture, and the exact position I stand or other positions.
I tried the skills available in this job, given by the developer on this job. For other skills, I have to acquire them in various ways, and that includes what weapon I use. If it's different from the weapon I'm using, it can't be used. So, the skill that I might get, must match the weapon I use. That way, I can use it.
The standard skills, which already exist in this Assassin job, are those that can be used with various weapons. Also, even if I'm without weapons, with only my hands, they can still be used on my character.