@Dash-Dash  
I am guessing that he used some sort of digital pen to make the strokes. Practice probably did it. I have seen streams of artists and they undo until the line comes out just right.

Hey, I really need your help, if you digitally drew that picture, how did you draw such amazingly straight line? I use gimp but unfortunately the colors aren’t very vagrant and it’s hard to use, I really want to know the art program you use, and how you drew the lines like that, this is a amazing picture, and I hope I can draw like that someday when I’ve learned my mistakes and corrected them, I’m just sick of making mistakes, I guess I make mistakes because it’s hard for me to visualize the shading and position of things, but that depends on style too, so I don’t know, if you can somehow explain this to me, it would be really nice of you to do so.

Awww-barf-wwwwww.