3q wowo!
陈河成: Sure no problem

"How do you make the lines so smooth" would be the right way
And to answer your question, I used Easy Paint Tool SAI. Its a Japanese painting program that is like Corel Painter light. The brushes have a bit of tracking on them like in Painter, so that you can get a smoother line. I combine this with Photoshop when I paint/draw.
At the same time, when you make the stroke, you still have to flow smoothly with your hand, or else no software will help you to get a smooth line. You should make the lines with your whole arm, not just your fingers. Use your arm, shoulder and wrist loosely, and have lots of confidence when putting down the line. What I did was take the silhouette (The all black painting in post #236 (see the first black silhouette there?) I took that, then set the opacity to about 35%, and took another layer and drew the lines. That way I knew where most of the lines should be, so I could apply them with more confidence, rather than guessing.
Hope that helped, and hope you understood! If you dont understand something, tell me, and I will try to explain in Chinese.