Using a fairly small stitch and the correct tension for your sewing machine, stitch the darts on your front and back leg pieces if you are using darts to fit your jeans to your doll's waist. When sewing small pieces of fabric, it is sometimes easier to stitch them on top of a sheet of paper, catching the paper in the stitching as you sew the fabric pieces together. This helps prevent the fabric from getting tangled in the feed dogs on your sewing machine.
If you want to see stitching on the base of the legs of your jeans, turn over the hem at the bottom of the jean legs and stitch it on the right side of the jean. If you prefer, you can stitch both hems on a single leg at one time after sewing the outer seam of the jean leg together, then opening out the fabric, turning up the hem, and sewing the hem on the front and back leg in one process. After sewing the leg hem, you can stitch the inner leg seam.
With the darts sewn, lay one front leg jean piece on top of a back leg piece so the right sides are in the middle touching each other. Sew the inner leg seam from the top of the leg down to the bottom, keeping the center seams free. (stitch the leg hem now if you want) Sew the outer leg seam from the waist to the bottom of the leg.
Tie off any loose threads, trim the seams to 1/8 inch and treat them with fray check. Test fit the jean leg, wrong side out, to your doll to make sure you can fit the leg over the doll's foot. Check to see that you have enough material for the center seam and the back seam for your doll's jeans. If you will be making an elastic waist, turn the excess fabric at the waist over and mark where the waist will turn as shown in the photo. (you can mark the turn with a bit of chalk or hard bar soap. Remove the jean leg from the doll.