To begin sewing a pair of doll jeans for any size or scale of doll, first make a Basic Pant Sloper for a Doll to fit your particular doll. The sloper should be made from lightweight interfacing, and will give you the fitting information you need to make a basic jean pattern.
When you have the sloper pattern completed, analyse the fit which will work best for your doll. Will you be able to point the doll's foot to put on the jeans, or will the jean leg need to fit an extended non moveable foot? Will you be able to give the doll a back opening large enough to pull the jeans on with a back closure, or will your jeans need to have an elastic waist to fit a doll with a very small waist compared to its hips. How much stretch will your doll jeans need? Should you try to find lightweight stretch denim so that the jeans can be fitted tighter than would be possible with woven material?
When you have decided what pattern adaptations you need to make for your doll's jeans to fit on and off easily, lay out your basic sloper pattern on a larger piece of interfacing or paper and trace around the sloper, tracing the lines of the waist darts as well. If you are making jeans with an elastic waist, you will not need to sew the darts, but you will need to add the extra material at the top of the dart into the waistband. If your doll has ball joints at the ankle, adjust your pattern by adding a minimum 1/8 inch seamline around your basic sloper, on the top and sides of both the front and the back sloper patterns. If your doll has fixed feet or shoes in place, you will need to make the bottom of the jean legs wide enough to fit over the doll feet.
Leave at least 1/4 inch hem at the base of the legs, unless you will be working with already finished seams on your fabric. (I used the hem on the shirt tail of my denim shirt as a finished hem for the base of my pant legs.) Mark a line down the center of your pant leg patterns from the point of the sloper dart. This will give you the line for the fabric grain.
Add a slightly wider seam line to the back center seam edges of your basic pattern. You will need to turn the edge of the back center seam into a fine hem in order to make a pair of back opening jeans.
At their most basic, you will have a pattern which is very similar to your sloper pattern, perhaps with a wider leg at the base of the leg, and a marked 1/8 inch (or larger depending on your doll size) seam allowance all the way around the pattern, except for the base of the leg, where you will need a slightly wider hem.
When you have pattern pieces drawn on paper, cut them out and check them against your doll or an existing pair of pants which fit your doll, to check your pattern. The first pair of pants you sew will be a test for your pattern, so don't expect it to be perfect the first time!
When you have a pattern you think will work, label the pieces with front and back and the doll's name. You can reuse these pattern pieces many times for many basic pant styles.
Cut out two front pieces and two back pieces for your jeans, laying your pattern on the fabric so the grain line on the pattern matches the grain (direction of the fabric lines) for the fabric. In the photo above you can see that one leg (on the left of the photo) has been correctly cut with the grain, while the front of the leg on the right has been cut off grain. See how the lines of the threads do not run straight up and down the fabric. I did this intentionally so you can see how the difference appears as you sew the jeans. Cutting fabric off grain will cause seamlines to stretch and fabric drape to be off grain when the pants are fitted. In the size of doll I was working with it wasn't too noticeable, but it is a good idea to keep fabric grain lines straight whenever you are making pants.