In this post we are going to explore one method of manipulating a photo to look like it has been sketched by pencil. We will use Adobe Photoshop.

First we duplicate our layer in the layers palette (Command (Mac) / Control (PC) + J).
Then desaturate this new layer by choosing Image > Adjustments > Deaturate.

Now we duplicate this layer (we will have three layers).
Next we choose Adjustments > Invert on the topmost layer.

Then change the topmost layer's blending mode to colour dodge.
The Image will look bad now but don't worry we will fix it in the next step.

Choose filter > blur > gaussian blur.
Apply as much blur as needed to bring the detail back into the image.
Every image will require different amounts of blur.

Select the top two layers in the layers palette.
Choose Layer > Merge Down

Change the topmost layer's blending mode to luminosity.
Finally we need to adjust the opacity to a setting which looks the best

Below is the final result.