Open up your image you want to turn into a sketch, im using an image of jessica alba. First thing we need to do is desaturate it do this by press "shift + ctrl +u" or goto "image > adjustments > desaturate".
Now goto "filter > stylize > find edges".
Now press "ctrl + L" to open up the levels palette these are the settings im using, but play around with them, also adjust the output settings change the lightness of the image.
and heres mine.
