If the wig has a skin top that you want the streaks to 'grow out of', use Katie Bair's back parting tutorial that Ani_BEE linked to. (You don't want to sew in that many extensions, of course, just a few tiny strands of loose extensions here and there.)
If the wig doesn't have a skin top where you want the extensions to be, get yourself some wefted extensions and simply sew them to the undermesh inbetween the existing wefts.
For a more natural look, you may want to do both, so that there are subtle highlights all over the wig and not just on the top or in the bottom layers.
Last edited by Kukkii-san : 01-07-2010 at 03:22 AM.