I don’t know how much of benefit I will provide to anyone by showing you how to clean your background, but I guess it doesn’t hurt to show how to do it anyway. There may be better methods, but this is the only one I know.
This method applies to those who colour in a crude manner (such as yours truly) and would like to have a separate background, like adding a stock image or logo or something beneath. Those who systematically colour by separating each element in proper, labelled layers with profuse grouping, or those who cell shade may need not apply. That’s because most likely your drawings are sparkling clean and spotless.
I have respect for people who have such patience.
By the way, you need photoshop to do this, it’s stated in the tutorial albeit with a typo (but who cares. I’m too lazy to edit this for the 136448th time).
You can download Adobe Photoshop CS2 for free at the official site, so..