Pants are CONFUSING. Really! Most people see a pants pattern and start backing away slowly. Worst of all, most pants patterns don't fit together.

I'm hoping I can make it a little better for you. This should at least give you an idea. Once you understand pants, they're probably the easiest thing in a doll's wardrobe, even easier than shirts.

You will need:

A pants pattern -- and you're probably going to need to buy it. More on this below.
Cloth -- probably about half a yard of material good for pants, like corduroy or denim.
Elastic -- For the waist band. (Someone is going to come back at me and ask, "You don't make pants with a fly?" No. I don't. You know why? Doll pants with flies are far too much effort for an end result where the doll's butt falls out of the pants every time you move her legs. See: Sparkily Tunic Outfit. The elastic waist band will keep your doll from mooning the neighbors, trust me on this.)
Sewing supplies -- Needle, thread, scissors, so forth. A sewing machine is good for this.

On pants patterns: About 99% of pants patterns suck and will not fit the first time you make them up. It might be best to make the pattern once out of cheap cloth, figure out where to make changes, then try it again.

Since you'll ask: the best pattern I've ever found, hands down, is from a Butterick set of patterns. It's B4089. Go buy it -- everything fits perfectly, the legs are just about a half inch long. It has some other really great patterns in there too, so it's totally worth the money.

I've noticed that the pattern in Doll's Dress Maker by Venus Dodge needs to be altered to have more room in the seat. I can't remember, but the leg length might be too short.

Don't even bother with a Simplicity pattern. It won't fit together. Their pants patterns look more like modern art than pants.

I traced the pattern from Butterick B4089 onto some notebook paper. Mine looks like this.

If yours doesn't look like that, there's a problem right there. (Yours may look like that folded in half, and that's okay.)

The bottom half of that pattern is the legs, and the top half is the part that covers the waist. The little hooks on the side are the part that fit around your doll's underside. (Raise your hand if you've noticed Bean trying to come up with polite terms for a doll's neither regions. Everyone? Good.)

You can tell what end of your pants you're working with by looking at those hooks. They point up toward the waist. The legs also should be slightly longer than the top part.

Cut two. If your pattern is folded in half, cut on the fold like the directions say. You should end up with this:

Press and hem the legs.

Now, place your pant halves so that the right sides are together. See where I'm pointing? We're going to sew between my fingers, from waist to inseam. Don't sew around the legs.

You should end up with this:

Press a casing into your pants. Wet the pants at the waist, turn them down once, press, then turn them down a second time and press. Make sure your casing is big enough to run elastic through.

Now, we're going to run elastic. Cut a piece of elastic slightly smaller than your doll's waist line. On one end, I stuck a pin halfway through. On the other end, I clipped a special tool called a bodkin -- but a safety pin will work if you don't have one. Slip the elastic into one end of the casing, and pull through to the other end.

Pin it.

When you finish, make sure that a small bit of elastic sticks out of both ends of the casing. Very important!

Turn your pants right side together again. See where I'm pointing? We're going to sew from the waist band, over the elastic casing, to the inseam again. Don't sew on the legs.

You should wind up with this. Stretch the elastic to make sure it holds -- if it pops out, take your last seam apart, dig the elastic out, and put it in again. It has to survive a stretching if it's going to stay on your doll.

Resist the urge to use your pants as a delightful party hat.

Instead, turn your pants so that the right sides face each other, but this time, fold it so that both of the inseams meet, and the hems meet. It should actually look like a pair of pants at this point. Start at one hem, sew up to the inseam, then sew from the inseam down to the other hem. You'll be sewing in an arc, up one leg and down the other. Turn.