A long time ago somone asked me to post a tutorial about how I work. Here it is!
I start with a sketch, then I ink it.
After scanning in low resolution, I turn it into hard pixels and transform it in vectors.
From vectos, I export it as a high resolution image, which is ready for colors.
That's how I clean little useless spots and make the traces smooth. That also allows me to make poster size images from little doodles.
The programs I use are Gimp and Inkscape. They're light, easy to find and for free.
If you know an easier way to do it, please tell me.
Hope you like it!
Link (c) Nintendo