Hi!
After you scan it, open the file with Photoshop and start drawing every pen-made line with the line tool. It will make lots of layers because this tool make only straight lines and you will need to draw maybe 1000 of them to finalyze your draw. Then, what you have to do is to rasterize all of them and pressing Ctrl+E all the time until you get a single merged line that you will call "pattern".
After you finish marking all the lines, you won't need the original file anymore so you can delete it. Now you will have a clean draw on a white background. From here you need to keep adding colours and shadows by layers. It's more easy than it seems, but explaining it here is so difficult.
I strongly recommend you to find Photoshop tutorials on how to work with layers.
Zai jian!
Ashifechi
這是一個外國朋友傳來的...
可是我對英文不熟,不好意思能幫我翻譯一下嗎?