How do you give jeans the look of vertical lines (streaks)?

Sandpaper and some rubbing.

bleach.

or you could go out and buy some ‘stonewashed’ (thats what it’s called) jeans

You really can’t duplicate the effect in a homestyle washer. If you fold the legs accordion style and tie tightly with string and then add bleach to the wash you might get something similar, but most likely you’ll get an effect you don’t like and will have ruined a pair of jeans. Like Joey said, these could be stonewashed. So if you’ve got some lava rock or pumice stone and a stainless steel basket in your washer, have at it!