Bunnystick has several great tutorials for using a basic paint program like Microsoft Paint to create pixel art and cartoon dolls. Includes color replacement, tracing bases, and shading, among other tutorials.
"First you have to choose a base and a pallette. If you have a very small base, you would only need a pallette of three or four colours. This base is fairly large so I used five colours. I labelled these A, B, C, D and E for easy reference. Also make sure that your shades are not too far apart, and that they are not too bright." -taken from the web page
"When you come to think about it, a palette is basically just a gradient with a reduced amount of colours. For this tutorial, we are using a technique on which I came across on DeviantArt for making gradients in MS Paint."
"Basemaking is an integral part of dolling, and after you've dolled a while, you might feel compelled to create a base. Many dollers use fancy programs and graphic tablets to create bases, but it's very possible to make a base with just MS Paint and a scanner."
"This is not exactly a doll tutorial, its not exactly a tutorial on how to use MS Paint either. It's just a collection of things which I think every MS Painter should know about the program. I bet you that there's something you didn't know in here. . . unless you're a total MS Paint expert!" -taken from the web page.