To counterpoint this, personally I've found if I jump straight to pen without having worked out each line pretty close with under-sketches, the end result suffers because my first attempt usually isn't very good. I like to build up form with a coloured pencil, which helps me work out the silhouette and understand where everything is going to go. Then I work out approximate lines with a B pencil and look to make sure I got all the proportions/anatomy right, and finally make my final lines in pen.
However!
I can see the value of being able to go straight to pen, as I guess all your working out would be done in your head which would be a lot more efficient. I just prefer to do my thinking on paper haha. Try find what's best for you, there's no right answer, and even then your answer may change.
Say to start with you need the extra steps of working things out, but then you start to be confident enough to skip out then before you know if you're banging out whole rooms full of people in one pass like Kim Jung Gi. Alternatively maybe you struggle through going straight to pen and you get really good at doing that, but then you need to start doing really complicated scenes or costumes that absolutely require some planning.
Not to say that either off those are specifically your goals, just different strokes for different folks and to do whatever you wanna do gosh
