Doing denim is pretty hard when you have Paint. I have a little help from Photo Studio 2000 but nothing beats Paint Shop Pro. But for those of you who have to rely on MS Paint like I do, or even if you have PSP but don't want to bother with all that difficult shading, here's the way to do it ;)

First, start with the doll you want to draw the jeans on. I'm using a base from Guinevere's Dolls. Get a handy pallette ready of shades of blue, from dark to light. It's best to choose blues that are more on the greyish scale, since denim is naturally not intense. Choosing the darkest shade, draw the outline of the jeans. It's far easier to use the Brush tool, set on the smallest round pixel, because unlike with the Pencil tool you can actually see what you're doing :P

Clean up the jeans' outline. Using the third shade from the top, fill in the jeans with the Bucket Fill tool.

Now, pick the second darkest shade. Draw where the darkest parts of the denim would generally lie; along the sides of the legs, in the slight folds at the hems, and in the waist and crotch area. Don't forget to make details such as the fly, beltloops and pockets (you can skip any of those three if you want).

Take the fourth lightest shade and start making highlights in the jeans; on the front of the thighs and on the waist details, on the knees, and down. Your doll should now look like this:

With the fifth lightest shade, carefully accentuate the highlights. Using the third lightest shade again, draw random "noise" speckles on the jeans close to the light areas. I zoomed in at x200 to do this, but you can zoom in closer if it's easier for you.

Now, taking the lightest shade of all, zoom in close to the jeans and carefully place the last highlights. Depending how big the light areas are on the jeans, this could give them a spiffy faded effect. You can also skip this step if you think it makes the jeans look too shiny.

Your doll should now look something like this. Congrats!

You can follow these steps to make a denim skirt, too. And experiment with the shades of blue! Feel free to make them darker or lighter as you want ^_^