You can find some actions in adobe exchange that do it for you, and you can study them... or you can do a google search on sketch photoshop tutorial and I'm sure you'll get plenty to keep you busy.
Peace,
Tony
<http://www.retouchpro.com/forums/showthread.php?t=4058>
<http://www.retouchpro.com/forums/showthread.php?t=4376>
Note: You will need to register at the site in order to open attachments.
This one, too: <http://www.trimoon.com/html/tm_sketching_tutorial.html>
HTH...
~DannyR~
Here's one way:
Pencil Sketch
1. With the original image open, duplicate the layer. =(Layer 1)
2. Desaturate Layer 1 (Image/ Adjustments/Desaturate)
3. Duplicate layer 1. =(Layer 2)
4. Invert layer 2. (Image/adjustments/invert)
5. Change the blending mode of layer 2 to Color Dodge.
The display will turn completely white.
6. Apply a gaussian blur to layer 2. The amount of blur will
determine the width of the sketch lines.
You can try different blurring tools. eg median, multiple cycles of
despeckle etc.
You can now delete the original layer or you can move it above Layer 2 and
reduce the opacity to get a watercolor look to the sketch.
The tutorial is named Photo To Sketch In 60 Seconds Flat.
Works VERY well.