This project assumes you already can doll pants.
Your parents (grand parents?) totally had these, I promise. This is historically accurate for Julie and Ivy.
In addition to everything you need for pants, you need:
Cloth -- Some sort of print cotton.
While we're at it, since we're making jeans, use denim to make the pants! Don't cheat and use blue cotton, that's just stinky.
In addition to the two denim pant pieces you'll be cutting, cut two isosceles triangles made from cotton. They should be about as tall as the measurement from your doll's foot to somewhere around her knee.
Hem the short side of the triangles and the bottom of the pant legs. Fold the triangle in half and use it to measure a line down the center of each pant leg.
Cut the line ever so slightly shorter than the length of the triangle.
Pin each long side of the triangle to the side of the cut you just made.
Sew it in.
Press it, then finish your pants. Enjoy!