Vector Graphic

1. Choose Scan

Choose a worthy scan you can vector. It's a waste of time if you use a clean scan. I used this Mai HiME scan with Mai and Natsuki (Click on thumbnail to see a bigger version of the scan).

2. Outlines

There are many possibilities to start your vector. The easiest way is to make black outlines. But you can also make them colored, however, it's a little bit harder. No outlines can also look good. If you want to start like this, then skip the second step.

To avoid unregular lines change the scan's opacity to 50%. Then zoom as close as you need to. Click 'D' to change to black/white colors. Use the pen tool . Draw the outlines about 1px and be sure to make them very close to the scan. Tip: Hold it longer and strech the pen and you can make nice round lines.

3. Shading and Highlights

Now you have the outlines in many layers. To merge them together just delete the scan and go to Layer -> Merge Visible. I suggest to save the outlines before the merging and save as a new file.
You can make the shading and highlights similar to the scan or make your own colors. I'm doing my own shadings but for this tutorial I used the other technique, which is easier, too. If you're able to make your own shadings just start with the base colors.

Start with the shading and highlight parts in a layer below the outlines' layer and above the original scan (use pen tool all the time).

4. Color

Go to the scan's layer again and start with the color below the shadows.

5. Finished

It's really important to take your time and not to hurry with the vector. If you have worked properly, you would have now a clean vector. Delete the scan and merge the outlines with the color. To check if you have no parts missing, make a colored background with a color like #FF00FF. Now you see all holes. If there aren't any, you have done a very good job. (Click on thumbnail to fullview the finished vector.)