First open the image you would like to convert into a pencil sketch and erase the background:
Next press CTRL+J to make a duplicate the layer. Then press CTRL+U and lower the saturation to reduce the complexity of the image:
Press CTRL+J again and make the image a negative by pressing CTRL+I:
Then make the blending mode of the new layer to Color Dodge:
You won't be able to see anything, so to view the lines or strokes apply a Gaussian Blur at the top layer as according to your needs:
Your final result will look something like this:
If you really want to beat out Leonardo, try tricks like painting only some parts of the top layer with a new color. Or, color just the lips or eyes of a portrait by selecting the area and applying some hue with CTRL+U.
More Tutorials: 
