Start a new project.
This is the Original Photo used for this tutorial.

counting house

Choose the biggest possible brush:

apply all settings steps 1 and 2

Select the «Autopaint» Tool:


Now apply it over the whole image:

apply to whole image

This is the result after 50 steps:

counting house - step 1

This is the result after 200 steps:

counting house - step 2

Now, make smaller the brush size, and apply it over the whole image.

apply all settings step 3

counting house - step 3

Finally, choose smaller values for Width and Height, and apply it manually over the whole image:

before sketching settings

This is the final result before drawing the sketch lines.

counting house - step 4

Now select the FreeHand Tool and draw the sketch lines over the most significant contours.
Keep in mind the following hits: