This is a solid case of “anime artists who don’t know how real-world clothing works.” There are two things you can do, aside from changing the fit of the jacket so it’s a literal rectangle, to imitate this look, though neither will be perfectly screen accurate because that’s impossible:
– Make the jacket out of strips, but flare them towards the bottom. This would be a lot like making a gored skirt, only the gores would extend to the top of the jacket, and there would be a lot of them.
– Make the jacket out of something more solid and flare it, for the shape, but add the lines artificially. You can do this by painting them on, or by sewing them basically as pintucks, with the tuck on the inside of the jacket. With this method, you can have straight lines instead of flared lines if you choose, but the rectangles won’t all line up perfectly at the front and sides because of the flare, and you’ll get some lines at a bit of an angle/meeting at points at the sides. I’d personally use this method, and start the lines so the center front stripes are straight, and the center back stripes are straight (if you aren’t shaping the center back seam), and then having them angle more towards the sides, since this would be the closest illusion to the official artwork, rather than having the stripes angle at the front. If you do this method, you can shape the bottom hemline on the solid piece and line it rather than using individual strips. (I suppose you can also make “fabric” out of strips and cut your pieces out of that)
I hope that helps! Good luck :]