Creating grunge paths manually using the pen tool

In this simple approach we will essentially create random paths with the pen tool and then intersect the paths repeatedly to create a look that does not look man-made.
First, select the pen tool and draw random shapes. Select the shapes, remove the stroke and add a background colour. The steps are shown below.

Now, drag one of the shapes on top of another. Select both paths and go to [Path -> Intersection]. After applying the intersection to the paths, you will get a new path that looks a little more random than the other two. The below pictures illustrate this process:

You should experiment with different hand-made paths to see which combinations work best. For example, here are the results of combining our resultant grunge path with other types of paths to create a second grunge path.

Applying the grunge paths to the original illustration

We will be making copies of our paths, rotating them and positioning them above our illustration. However, the paths' background has to be the same as the image's background, and we will not see the paths as we move them. This is why to apply the grunge paths, we need to put a black background behind the illustration.

First add a new layer by going to [Layer -> Add Layer...]. Move this new layer to the very bottom by going to [Layer->Layer To Bottom]. Finally, with this low layer selected, draw a black rectangle and position it behind the illustration, lock the layer. Place the grunge brushes on top of the black rectangle and set their colour to white (the same as the image's final background colour). The result will look something like this:

We copy, rotate and position the white grunge paths above our illustration.

Now we remove the black rectangle by either deleting it or making its layer invisible. Note how the grunge brushes disappear and the illustration looks as if its been beat up. I have also added thin white lines to simulate scratches.

The paths made using this particular technique don't look very convincing as scratches because each bit of the path is fairly large (and this is a constraint of the pen tool). Knowing the above steps, though, you will immediately understand the next technique.