1) Open an image.

2) Create a copy of the background layer (Layer->Duplicate layer)

3) invert it (Image->Adjust->Invert).

4)Convert the image to Grayscale (Image->Mode->Grayscale) and choose not to flatten the image.

5) Double click the layer to open bending mode options palette. Set layer mode to "Color Dodge". This will at first make the layer look almost completely white.

6) Now, to bring out some pencil strokes apply the Gaussian blur-filter (Filter->Blur->Gaussian Blur) and set "Radius" to somewhere between 1.5 and 5.0 pixels.( If you set "Radius" to a higher value the image will look rather more like a pastel painting than a sketch.)