It's easier then it looks, really. I think what makes everything smooth is not having a plan. I don't pre-draw anything, i just make it up as i go. I feel like if you were trying to retrace lines that you'd move slower and your lines wouldn't be as whymsical. Move quickly and the lines will already be smooth. Doing the longest lines while the surface is blank gives you the freedom to let them go wherever they want. Depending on the art you do, you might be better at this then you think. I've done entire cars like this and the best i can say is don't pre-plan anything and move gracefully. If you ever have any ?'s, feel free to ask!