You would need to use a bleach on the jeans and let them set for a couple hours. For small, precise streaks, the best thing to use is a bleach pen -- these are sold in the laundry products section of grocery stores. For larger areas, use a spray bottle filled with ¼ bleach and ¾ water. Stuff the part of the jeans you are working on with old towels so that the bleach solution won't go all the way through, and apply the bleach check after 10 minutes and apply more if needed. Once the desired amount of streaking has been reached, wash the jeans immediately. Failing to do this will cause the bleach-soaked fibres to weaken.