Back when I made my first stripe skirt, I was pleased with the matched side seams but had no idea how I did it. Well, dang if I didn't find some photos today that I snapped in process. Yay me!
However, cannot fully explain how I matched the stripes so perfectly but will direct you to notice the bottom hem edge at right angle edge of fabric and slight diagonal cut on side.
My mom laughs at me because I am teaching myself how to sew on the most difficult of fabrics - silk AND stripes. She says that if I can do that, then I could probably split atoms. What she hasn't realized is that the huge stash of silk is my PRACTICE fabric.