You can either sew them on and revolve the piece of fabeic (the stripe) with a zig-zag stitch at a low step (0.5mm per stitch or so). Or you can sew them in: Cut out the place where the stripe is supposed to be (after adding around 1/4 inch of seam allowance) and cut out the stripe plus seam allowance with the darker fabric. The seam allowances should overlap when you put the stripe on top of the cut. After that you can sew in the stripe piece into the lighter fabric, just like you would sew together any other piece of the plush. Just keep in mind the exact number of inches you use as seam allowance. Or else it'll get all wobbly and uneven.

Easy thing explained complicated. xD

Sorry for the late reply! There are more ways to sew the stripes.